# 72% prop slip!



## InlandKid (Aug 29, 2006)

Last year I bought a 94 smokercraft 161 pro mag, I fixed it up over winter and bought a brand new suzuki 20hp for it two months ago. Last week I took it out for the first time and ran it for 5 hours following the break in procedure. When I was able to run full throttle the most I could get out of it was 7.7mph playing with trim positions ,the cavitation plate is half inch lower than the bottom of the boat. I ran a prop slip calculator and it said 72% slip is occurring. Here's the numbers, gear ratio 2.08, pitch is stock 10, rpm 6100, max speed 7.7mph. I'm thinking I need to get a four blade prop and drop my pitch, anyone have a 20hp they are running on a 16ft? What prop are you running and what speed do you get?


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## chrisrf815 (Jul 6, 2014)

Something is seriously wrong. I had a 16 foot crestliner, comparable boat to yours, with a 9.9. With me and me big buddy and all our gear, we were able to go 10mph. I dont know about what my rpms were. Your rpms sound about right, but they may be low. 
My first thought is check your throttle linkage and make sure they are actually allowing the carb to open all the way to "wot". The things in the front of the outboard were there is an adjustment nut that connects to throttle cables.
You should be able to hit high teens to 20mph no problems. 7.7mph is super slow for that setup.


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## InlandKid (Aug 29, 2006)

Max rpm for the motor are 5300-6300 is what suzuki says. Took hood off and just checked the throttle body, appears to be all the way open when I push the throttle forward. I went on Solars prop calculator and for a 16ft aluminum console boat total weight at 1000lbs it recommended a 8 pitch 3 blade prop on my motor. Would having too high of a pitch cause the prop to "slip" and not push the water?


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## chrisrf815 (Jul 6, 2014)

Too high of a pitch actually lowers rpms, lowering the pitch increases rpms. 
This might sound stupid but is the prop nut gully tightened


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## chrisrf815 (Jul 6, 2014)

i dont think an 8 or 10 pitch would make much difference on that engine let alone cause that much slip, i dont think the factory prop would underperform that bad


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## InlandKid (Aug 29, 2006)

I'll have to check the prop nut, do I need a special wrench or socket for that, and if so where can I buy one. Thanks guys. I was shocked on my speed.


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## chrisrf815 (Jul 6, 2014)

I got a mercury, its a 1and1/16" socket, i use a ratchet. But they make slecial prop nut wrenches. On my engine i gotta torque it to 50 ft/lbs


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## chrisrf815 (Jul 6, 2014)

west marine or any marine store will sell prop wrenches


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## sherman51 (Apr 12, 2011)

i've never used a prop wrench. just use a socket and 1/2" drive ratchet. and yes an 8 pitch will give you a much better hole shot. if you can get her on plane your speed will be much better. an 8 p 4 blade should be even better.


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## bustedrod (May 13, 2015)

take prop off and check that the hub isnt slipping. put it in gear and man handel the prop, both directions.


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## firemanmike2127 (Mar 17, 2013)

busted rod, I'm going to differ with your suggestion. I DON'T think that's a good idea at all. If the OP flips his impeller he'll possibly create a cooling problem. The flywheel should not be allowed to turn counterclockwise, which could happen if he 'manhandles the prop'. A far better solution IMO is to remove the prop & using a center punch mark the splined sleeve of the hub. (I usually do this on the front portion of the prop).Punch a second mark on the outer portion of the prop directly adjacent to the first mark. 
Then reinstall the prop & take it for a test run. If the marks 'move' he's got a hub that's slipping. I've been checking them this way for years & it's a pretty foolproof method. Mike


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## InlandKid (Aug 29, 2006)

I'll check the prop too but its brand new this was the first time it was on the water I hope the factory didn't sell me a motor with a spun prop hub.


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## Mickey (Oct 22, 2011)

It's a BRAND NEW MOTOR. The dealer should surely take care of the problem unless you bought it from a private individual. It may just be the angle the motor is set at.


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## $diesel$ (Aug 3, 2018)

Is it a rubber center inside the prop without a shear pin? Could be that center has broken.


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## sherman51 (Apr 12, 2011)

I'm not saying it has spun. but just because something is new don't take it for granted that its good. and 72% prop slippage seems a little high. and at only 7.7 mph you shouldn't be turning max rpm's. but if the prop checks good id go to an 8 pitch prop.

just a thought. you didn't get a shot shaft motor on a 20" transom. did you? the cavitation plate should be almost even with the bottom of your boat. if its to high it would cause all your problems.


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## bustedrod (May 13, 2015)

good point mike


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## wave warrior (Oct 2, 2005)

sherman51 said:


> I'm not saying it has spun. but just because something is new don't take it for granted that its good. and 72% prop slippage seems a little high. and at only 7.7 mph you shouldn't be turning max rpm's. but if the prop checks good id go to an 8 pitch prop.
> 
> just a thought. you didn't get a shot shaft motor on a 20" transom. did you? the cavitation plate should be almost even with the bottom of your boat. if its to high it would cause all your problems.


Says in post , cavitation plate 1/2" lower than bottom of boat........


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## sherman51 (Apr 12, 2011)

wave warrior said:


> Says in post , cavitation plate 1/2" lower than bottom of boat........


sorry, I missed that. its got to be the prop. a 9.9 would push it close to 7.7. a 20 hp with just him in the boat should plane off, I would think.


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## InlandKid (Aug 29, 2006)

Ok so I pulled the lock pin on the prop nut and checked with a socket and it was tight . Took prop off and marked hub, hoping to take it out Wednesday to check if the marks move.


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## firemanmike2127 (Mar 17, 2013)

What does the boat weigh ? I'm talking about the actual weight the way it is rigged along with your weight added in to the total. Also, is there a bow mounted T/M or any batteries stored up front ? Mike


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## InlandKid (Aug 29, 2006)

No bow trolling motor, one battery near the stern, 3gallons of gas near the stern ,boat sticker is 720lbs, I'm 152Lbs and my dad was with me he is 200lbs. Gear was one tackle box in the front maybe 8lbs and 4 rods.


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## wave warrior (Oct 2, 2005)

InlandKid said:


> No bow trolling motor, one battery near the stern, 3gallons of gas near the stern ,boat sticker is 720lbs, I'm 152Lbs and my dad was with me he is 200lbs. Gear was one tackle box in the front maybe 8lbs and 4 rods.


Haven't seen pics of the setup , but on many of my boats I've gained performance by moving battery and tank to middle of the boat , I would certainly think that 20hp would get you well into the upper teens ,even low 20's


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## sherman51 (Apr 12, 2011)

wave warrior said:


> Haven't seen pics of the setup , but on many of my boats I've gained performance by moving battery and tank to middle of the boat , I would certainly think that 20hp would get you well into the upper teens ,even low 20's


I would agree on speed. but its got to have enough power to get the boat up. i'd try the 8p prop.


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## 2120Tom (Jul 2, 2008)

8" pitch sounds to me as a thrust style large heavy boat trolling prop. I've had some 20-25hp over my years of boating and all had higher pitch props, 11-13" for all around use.
If the rpms' are there a faulty hub is definitely something to inspect.


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## bowhunter1487 (Aug 13, 2014)

My 16ft sylvan planes no problem with a 20 hp loaded with 2 guys and a good amount of gear and a full tank of gas. 8 pitch. Does about 15 mph. Solo I can get up to about 18 mph.


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## firemanmike2127 (Mar 17, 2013)

InlandKid, aboutmabout question re: weight. I was looking for the total weight of your loaded boat, not the information on the capacity plate. Reason being, if it's heavy due to waterlogged spray or poured open cell foam under the floor you'll be fighting an uphill battle. Just trying to verify that your hull is in the proper weight range for a boat it's size. Mike


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## InlandKid (Aug 29, 2006)

The weight of the boat from factory is 720lbs, no modifications have been made to it. Not sure how to figure out the weight other than that.


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## sherman51 (Apr 12, 2011)

InlandKid said:


> The weight of the boat from factory is 720lbs, no modifications have been made to it. Not sure how to figure out the weight other than that.


I believe a 8 pitch will let the motor rev at a higher rpm at slow speeds giving you a much better hole shot and getting the boat on plane. the boat is going to have to plane to get decent speed.


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## firemanmike2127 (Mar 17, 2013)

Take it to your local grain elevator. Weigh the whole rig & then subtract the weight of the motor & trailer to determine what the actual weight of the boat is. Mike


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## DHower08 (Nov 20, 2015)

Sounds like a spun prop hub. I had that issue once with a 9.9 thing could hardly move the boat more than a few mph. New prop solved the problem. That boat should be moving pretty good with that motor


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## firemanmike2127 (Mar 17, 2013)

I'd suspect a hub issue also but having never seen the OP's rig made me wonder about the total weight. I've seen a few hulls that were heavy & simply wouldn't perform. I have a customer's old fiberglass 15' Tidecraft bass boat at my shop now that is full of waterlogged foam under the floor. Older rigs that sit outside uncovered are especially susceptible to weight gain when old foam is allowed to get wet repeatedly. Mike


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## UFM82 (Apr 6, 2004)

Your initial post had a comment that immediately drew my attention. You said the anti-ventilation plate is 1/2" BELOW the keel. This incorrect. The plate should be at least 1/2" ABOVE the keel and this is why. When you are sitting still, you want to plate just below the surface of the water. This prevents the prop from sucking air in from above when you take off, thus the name "anti-ventilation plate". The keel of the boat will be several inches below the waterline of course. When you take off, the water pressure coming up against the underside of the plate prevents the prop from sucking air in and ventilating. However, once you are moving the keel rises and you want the plate to be at the surface of the water and here is why. 
1) Since it's not fully in the water, it cuts down on drag. 
2) Once you are moving the prop will no longer want to suck air as you now have water flowing over the blades. 
3) The water coming from under the keel will move "up" and push against the bottom of the plate, give you lift. If the plate is down too far it can dig in to the water and actually pull the keel down, especially if you are trimming up in an attempt to gain speed. The plate will be tilted down in the front when the motor is trimmed up and will be trying to dig in. That's like having a brake on and will contribute mightily to prop slip. When trimmed down it will be too deep in the water and cause drag, slowing your take off. 
4) Having the plate too far down will also cause heavy steering as the added drag will make the engine harder to turn. A 20 hp engine won't have as much of an issue as say a 150 but it's a factor nonetheless. 

Changing prop pitches without knowing your RPM can be dangerous as you won't know if you are running in your proper range. All outboards have WOT ranges and you need to be within that range for best performance. A 20 hp isn't going to be a speed demon but it should push your boat into the high teens easily enough. The 15 I had on a 16' Sea Nymph ran 16 mph you should be getting at least that. Move the outboard up on the transom until the bottom of the plate is at least 1/2" above the keel and run it. If that helps, you can move it up another 1/2" or until you start ventilating on take-off. If that happens move it back down 1/2" and leave it there. That should be a good spot for it. Steering will be light, the top speed should be right and the outboard won't be working as hard to push you down the lake.

Years ago Doc Lange, a known person on this board, was having serious issues with his new boat. Big, heavy aluminum Deep-V like a Starcraft or a Lund (can't remember the brand) and the dealer had installed the outboard waaaay too low. It was a pig. After he raised the outboard up like I outlined the boat came to life and was much, much better. Night and day was the comment.


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## InlandKid (Aug 29, 2006)

Took the boat out today and here's the progress and new info. Raised the motor 1.5 inches to put the plate .5 above the bottom. Marked the prop and hub and just checked when we got home and they still are lined up. Today I was able to get the boat to go 11.9mph and my rpms were 6600 on the tach which I'm starting to wonder how accurate it is so I ordered a new one. I may raise the motor another half in and try that next time.


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## UFM82 (Apr 6, 2004)

If the 6600 is accurate you need to increase pitch by a good bit. Is there a 10 or 11 prop for that? I think raising the motor was necessary as I said in my post right before you posted but 6600 is going to stress your new engine. Increase pitch and raise the engine like I said. You'll find the sweet spot and the boat will jump up on plane and scare you! LOL. Sometimes it takes some tinkering but you'll find the right set up. It's not rocket science but it is kind of. You are looking for the most hydrodynamic set-up you can find. The less motor that is in the water the better as long as you are pulling water for cooling and not breaking the prop loose. Remember also though that if you raise too much, it may be ok in a straight line but will blow out in a turn. If so, drop down a bit at a time to hit your spot. Good luck. 

You know it.


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## InlandKid (Aug 29, 2006)

I'm thinking the tach is off because factory wot is 5300-6300. Making me think that the rev limiter would kick in if I was above 6300 no? It wasn't on plane that's for sure. I had to have the motor trimmed out all the way to the last hole, if I trimmed it down any more I would lose speed.


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## DHower08 (Nov 20, 2015)

I run a 25 hp on my 16ft tracker and it's pretty heavy with 3 batteries full decks and enough tackle to open a store and I still run 23 or so by my self. I will look at prop tomorrow if I remember but I believe I'm running a 10x11 this motors not hooked up to a tack but by ear I would guess I'm running right at 6k


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## DHower08 (Nov 20, 2015)

Also if you get that motor to high it's going to suck air and cavitate. RPM will soar and boat won't go any faster as it won't be able to grab the water


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## UFM82 (Apr 6, 2004)

Does it have a rev limiter? If it does I doubt it will kick in at 6300 as you may exceed that figure a bit during normal operation and that's ok. You wouldn't want the limiter to kick in if you are buzzing along at 6300 and happen to hit the downside of a wave, right? It would be set at a number that would prevent the engine from revving to the point of damaging the engine. You can check the actual rpm with a rpm gun by putting a piece of reflective tape on the flywheel and running the engine. It will read the rpm and will tell you if the tach is accurate. If it is and you're turning 6800 you're already way over your rpm range and that tells you your limiter doesn't kick in at that range anyway. 


You know it.


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## sherman51 (Apr 12, 2011)

if the tach is right and your already turning max rpm's a 8 pitch will over tach at wot. if you do try an 8 pitch ou will have to watch your rpm's at wot and throttle back to max rpm's. and 8 will grab more water pushing the boat much faster at slower speeds hopefully getting you on plane.

back yrs ago I didn't know crap about the pitch. I had an 18' aluminum boat. with it loaded for Erie we had to walk forward to get on plane. I saw a going out of business sign at a prop shop. I stopped and the only prop he had that would fit my 80 hp mariner was a 15p. I bought it for a spare. after a couple more trips to Erie, I decided to try the new prop. the one I took off was a 19p which didn't tell me anything. put the 15p on and headed out. cleared the break walls and hammered down. the boat jumped on plane so fast I couldn't believe it. but at wot, it would over tach. so I would just throttle back to max rpm. if i had knew more about pitch back then, what i needed was a 17p. the lower the pitch the more low-end power you have. but your rpm's will be higher at wot.

I could tell you about the time I bought a 50 hp johnson off a houseboat with a 13p prop. I put it on a 14' runabout. I didn't have a tach. it would take off at full speed. but I kept blowing the reed valves. I figured out some yrs later the 13p was way to low letting me way over tach.


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## InlandKid (Aug 29, 2006)

I'm borrowing a four blade 8pitch to see if that will help push it on plane but keep me in my WOT rpm range. Hopefully I'll be able to try it on monday and let you guys know.


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## UFM82 (Apr 6, 2004)

If the prop you have now is a 10 and your tach is accurate you're going to over rev the engine. If the tach is suspect, fix it before you start messing with props. A 20 hp isn't going to show you big differences as it simply doesn't have the power to change things that much. Your initial post tells me you have a bad set-up issue beyond fixing with a prop swap. 
I didn't ask before- is this a bolted on engine or it it a clamp on? Is your transom tall enough to move the outboard up far enough? You don't have trim/tilt, correct? Tiller or remote?

You know it.


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## InlandKid (Aug 29, 2006)

UFM82 said:


> If the prop you have now is a 10 and your tach is accurate you're going to over rev the engine. If the tach is suspect, fix it before you start messing with props. A 20 hp isn't going to show you big differences as it simply doesn't have the power to change things that much. Your initial post tells me you have a bad set-up issue beyond fixing with a prop swap.
> I didn't ask before- is this a bolted on engine or it it a clamp on? Is your transom tall enough to move the outboard up far enough? You don't have trim/tilt, correct? Tiller or remote?
> 
> You know it.


Remote, clamp on but has bolt holes, I've played with the trim, if I want to move the engine higher I need a jack plate, I got a new tach in the mail today to install.


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## sherman51 (Apr 12, 2011)

when you try the 8p if the boat gets on plane watch your rpm's. if it starts to over tach pull the throttle back just enough to keep from over tacking. I did it for yrs on my 18' aluminum boat with the 80 hp mariner with a 15p prop I put on it.


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## bustedrod (May 13, 2015)

look at the back of your new tac it may have an adjuster for how many cyls. set it when ya put it in


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## UFM82 (Apr 6, 2004)

Um, if you have to pull back on the throttle to keep from overrevving you're underpropped. Just sayin'.

You know it.


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## DHower08 (Nov 20, 2015)

Just checked prop forgot all about this thread. I am running a 10.5 x 11 on a 25 go 2 stroke. On a 16.5 foot fully decked tracker and I run 23 to 25 depending on load. With 2 guys and all our crap around 20 or so


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## InlandKid (Aug 29, 2006)

Was able to get out today and here's what happened, I hooked up the new digital tach while leaving the old one hooked up. The old tach was reading higher by over 1000rpm. I tried the four blade 8 pitch prop today and my speed dropped to 7.2mph and my rpm maxed at 6230 with that prop. The boat still did not plane. I need to put the original prop back on and see what the new tach reads for the right rpms to see what direction I really need to go with the prop because the four blade definitely didn't work.


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## wave warrior (Oct 2, 2005)

Call prop doc in Cleveland , see what they say....


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## DHower08 (Nov 20, 2015)

Have you called the dealer you purchased from


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## InlandKid (Aug 29, 2006)

wave warrior said:


> Call prop doc in Cleveland , see what they say....


Did quick google search didn't find prop doc, sante marine propeller pops up is that the place?


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## fastwater (Apr 1, 2014)

InlandKid said:


> Took the boat out today and here's the progress and new info. *Raised the motor 1.5 inches to put the plate .5 above the bottom. *Marked the prop and hub and just checked when we got home and they still are lined up. Today I was able to get the boat to go 11.9mph and my rpms were 6600 on the tach which I'm starting to wonder how accurate it is so I ordered a new one. I may raise the motor another half in and try that next time.


When you stated you 'raised the motor' are you meaning you raised engine with engine trimmed in the all the way down position and you raised engine straight up off the transom until the cav plate was .5 above hull of boat...or...do you mean you used the trim function and trimmed the engine out until cav plate was .5 above hull?


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## InlandKid (Aug 29, 2006)

fastwater said:


> When you stated you 'raised the motor' are you meaning you raised engine with engine trimmed in the all the way down position and you raised engine straight up off the transom until the cav plate was .5 above hull of boat...or...do you mean you used the trim function and trimmed the engine out until cav plate was .5 above hull?


Raised the motor mount on the transom up.


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## fastwater (Apr 1, 2014)

Since your engine has been raised and you have a new tach, would do as you stated and put stock prop back on and try.
Your boat specs read stock boat weight is 720lbs. So most likely using the figure of 1000lbs may be light when loaded with extras bodies and gear. Think I would figure total weight at minimum 1100-1150lbs.
FWIW, my 1994 Starcraft Pikemaster 16'er has a stock weight of about 670lbs. I'm running a 25hp Mariner with 13p stock prop, mtr trimmed to 2nd hole and it will run 22-24mph heavily loaded to about 1300lbs total weight if not bucking a strong head wind. Comes out of the hole and planes great.
If you put the 10p back on with the engine raised at proper height, if you don't get at least mid teens out of it, I'd be doing as firemanmike suggested earlier and taking boat to feed mill or someplace that has scales and getting it weighed. Besides the possibility of being waterlogged...dont know what mods you did last winter but if you put new floor, storage areas, casting deck etc...that could have changed your stock weight a bit more than might be realized.
Would also be checking transom closely to make sure there's no cracks letting engine flex on acceleration.
The 10p might not end up being the optimal pitch for that rig but seems it should get it on plane and do at least mid teens provided everything is within specs.


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## wave warrior (Oct 2, 2005)

InlandKid said:


> Did quick google search didn't find prop doc, sante marine propeller pops up is that the place?


. might be called prop godz


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