41% slip so i lower the pitch, right?

[j5ffx]

Hi, i have a 14ft fish/ski (bonwitco 449) boat with a 1981 johnson 2stoke 35hp fitted with a new 10x13 prop. it will, when i can get it on the plane at WOT 5500revs do 20mph. sometimes it just seem to bog down at around 4500 revs. i used a slip calc and found my slip is 41% which seems high to me. should it be around 10, +or- 5.
am i right in thinking if i drop to say a 10x11 prop it will have a better hole shot and possibly top speed will inprove due to the fact i may get less slipage!!! more grip at the same 5500 revs
if anyone could tell me what prop size to get, i would be greatful.

 

[QC]

Re: 41% slip so i lower the pitch, right?

No. Something else is wrong . . . How are you getting your RPM, speed, gear ratio etc. numbers? Also, if you have 41% slip, you are either ventilating or the hub is slipping, and I don't think it's the latter . . . Give us all of the raw data. The 10 x 11 will improve hole shot, and maybe you are not getting the right RPM already. It will not fix slippage unless that prop hub is slipping. It might by accident, but something is causing the 41% slip or your numbers are wrong.

 

[j5ffx]

Re: 41% slip so i lower the pitch, right?

thanks for the input QC,
below is all the raw data

well i had the boat out this weekend and completed some test runs with it at sea in very calm conditions and adjusting the engine tilt setting to find the best setup. (2nd peg out)
Jimmbo gave me this gear ratio for this engine on another thread, he said it is 14:27 (.52)

Gear ratio: 14:27 (.52) i think this translates to 1.9 (the figure i put into a slip calc)
rpm: 5500 max wot read from a fully working teleflex tachometer
mph: 20 max measured with 2 gps systems
Prop 10x13 in perfect standard condition (no cupping) brand new
weight: approx 1200 lbs (everything included)
hull type: 14ft fish/ski with enclosed cuddy
Engine: 35hp johnson 2 stroke 1981 electric start all in perfect working order.

it pulls away well, upto about 8/9 mph but then very very very slow to plane and top speed, roughly taking 20, 30 or even 40 seconds to reach top speed! it actually got to 21mph @ 5600 on one run, but could not repeat it.

 

[j5ffx]

Re: 41% slip so i lower the pitch, right?

BTW:
the hull is clean, it only gets launched 1 day in every six weeks. so no barnicals etc:
so, do i have a problem?
should i lower the pitch?
am i missing something?

 

[MikDee]

Re: 41% slip so i lower the pitch, right?

it pulls away well, upto about 8/9 mph but then very very very slow to plane and top speed, roughly taking 20, 30 or even 40 seconds to reach top speed! it actually got to 21mph @ 5600 on one run, but could not repeat it.

Your hub is slipping, it has to be, new prop or not.

My first boat was a 16' Thompson lapstrake with that same prop, and a 1962 40hp evinrude, and would do about 26mph, unless your boat is alot heavier then mine was?, but I doubt it.

 

[QC]

Re: 41% slip so i lower the pitch, right?

Hold on . . . 26 vs. 21 MPH with 5 hp less is not an automatic hub is slipping. Easy to check though. Take a punch or something hard and scribe a mark from the prop to the hub. Run the boat, if the marks still line up, it ain't slippin'. There are very few definitives, we still need to troubleshoot this . . .

 

[steelespike]

Re: 41% slip so i lower the pitch, right?

Where is the antivent plate in relation to the bottom of the boat?Should be at least about even with the bottom.
Boat should run with about the rear half in the water?Does it seem to be plowing?13in pitch 20 mph 5,500 rpm something is screwy.Maybe double check the pitch.Have it measured perhaps an error in marking.
That 14footer is sure no flyweight Throw out the motor and gas and youve got a 1,000
lb 14 ft boat that must be loaded for bear.My 18 ft bare bones
aluminum 330 lbs,Merc 500 200 lbs gas and battery 100 lbs oars,pfds etc
50lbs. a little over half that 14 footer.

 

[j5ffx]

Re: 41% slip so i lower the pitch, right?

i dont think the hub is slipping because the engine would rev up, but it dosnt. it always gives the impression it is under load and trying its best.
i will how ever mark the hub just in case.
i should point out that this boat is one of the unsinkable type (foam filled) perhaps that is retaining some water making it heavier than i think. i did removed the drain plug and tilted the boat up a few weeks ago for a week but nothing really came out. so i wouldnt think that that would make a big differance.
the previous prop was also a 10x13 it had a few tiny dinks and a small piece missing and also gave very similar performance!!! making me think the new prop is a also a 10x13 as stamped on the side, but i will check it. the antivent plate is level with the bottom of the boat and it has doel fins fitted.
while testing the boat i tried moving weight around the boat as well as adjusting the engine tilt, so no its not plowing.

 

[QC]

Re: 41% slip so i lower the pitch, right?

it pulls away well, upto about 8/9 mph but then very very very slow to plane and top speed, roughly taking 20, 30 or even 40 seconds to reach top speed! it actually got to 21mph @ 5600 on one run, but could not repeat it.

This sounds like weight and possibly plowing to me as well, but I am still struggling with 41% . . . You are 100% positive that the "fully working teleflex tachometer" is fully working accurately? No sarcasm, and I am no OB guy, but it can be good and still inaccurate depending on how it gets its signal . . .

 

[steelespike]

Re: 41% slip so i lower the pitch, right?

Have you tried it without the Doel fin.They can cause problems including reduced top end. Still 13in pitch 5,500 rpm and 20 mph don't add up to me.
How would you describe the boat, fast bottom ,true deep V rounded bottom.
Just for the heck of it I checked some 40 hp setups.Obviously more hp and a different gear ratio. but weight was around 1,300 lbs as tested all 3 tests
were over 30 mph.How is your compression?Is throttle opening all the way?

 

[j5ffx]

Re: 41% slip so i lower the pitch, right?

yes, the tachometer is working fine, i know because i did have false readings due to a faulty rectifier. since then the engine has had all the electrics checked out, compression was 110 on both cylinders.
btw: now i have the correct gear ratio (above) it is actually 44% slip.

Newsflash: the hull is a Dromedile hull, "no i havent heard of it before"
quote:- designed in sweden and able to do both displacement and planing work equally well....

equally well? that sounds like some kind of compremise to me!

i can confirm it is very good in a choppy sea at low speed. but, it now sounds to me like there must be more hull in the water than a average ski/fish boat designed just to plane well. plus being on the heavy side that would mean even more hull in the water, wouldnt it?
so, surely there must be more resistance to over come than average.
so, a lower pitch prop than the stardard 10x13 that this engine was sold with
would improve hole shoot and even if the slip remained the same, top speed would only drop 18mph, if it reduced the slip to 30% the speed would increase to 22mph i at 20% slip it shows a top speed of 25mph.!!!
well that makes sense to me, lol. but im no expert that why im here asking you guys....

 

[j5ffx]

Re: 41% slip so i lower the pitch, right?

no i havnt tried it without Doel fins, i will try that.
hull is deep v up front, swallow v at rear (Dromedile hull)
throttle opens all the way and the link and sync is setup 100%

 

[QC]

Re: 41% slip so i lower the pitch, right?

Possibly water logged and a Doel fin . . . hmmmmmm, I think we are getting closer. Missed that.

 

[CharlieB]

Re: 41% slip so i lower the pitch, right?

Doel fin may help raise the rear of the boat a bit quicker BUT the additional drag from the added surface area is going to further REDUCED top speed at WOT and cause your slip % to RAISE.

Your problem is not enough HP for the weight.

Increasing blade area or adding cup to increase 'traction', reduce slip, will reduce WOT RPM. In your case, your engine is performing up to rating, reducing rpm, lugging the engine could cause harm to the motor.

Reduce pitch AND increase blade area, better 'traction', reduced slip and the reduction in pitch should allow the engine to pull it and maintain rpm

Anyone make a 10 X 10 4 Blade?