Prop for 20hp Yamaha

[yama33]

Hi all, I need some help propping a 20hp yamaha 4 stroke on a tiller boat. Details as follows:

1) Performance issue you are trying to correct:

Very slow to plane with 2 people or not at all, more top end if possible

2) Current prop manufacturer, model, aluminum or stainless as a minimum.
Yamaha aluminum stock

3) Current prop diameter and pitch (required).
Stock Yamaha 9 1/4 x 10 - 3 blade

4) Wide open throttle RPM and speed with an average load (very helpful)
** RPM's taken with tiny tach - wrapped around both spark plug wires (when wrapped around 1 only gives half the RPM's at WOT)
1 person - 6000rpm - planes fairly quick - 24mph
2 people 1 large, 1 small adult - 5700rpm - very long to plane - 22mph
2 large adults - less than 5000rpm - won't plane - 12mph

5) Engine/drive make, model, year, and HP
Yamaha 20hp 4 stroke

6) Boat make model, year, length and weight
14.5ft tiller vhull 500lbs dry weight

I'm wondering where I go from here - alone it runs good and at max RPM so can't really drop pitch. Would a 4 blade help? WOT rpm range is 5500-6000.

Also, motor cav plate is also 2.5in below bottom of boat. I might raise it via mini jacker or building up transom but not sure that will help that much with the planing but should raise RPM and top speed a bit. Would prefer to change props rather than start drilling transom etc. But, if I have to, I will

Thanks

 

[Sea Rider]

Re: Prop for 20hp Yamaha

Welcome to Iboats,

That's strange, normally Factory delivered props performs right on light to medium loaded boats, so you have the non programable TT right ? I would begin checking lower leg-transom height. Float boat alone, trim engine to have a perpendicular engine (90 deg) with respect to w?ter level.

Place a boater up front, go wot prefferibly on flat calm, no windy water cond, pull your head out transom and check firstly if having over or back transom water splashes. If not check where is water flow height passing with respect to small upper def plate and anticav plate. Visually is the only and fastest way to check if engine-transom has a perfect match. A correct height seated engine can gain as much as 200-250 more rpm that just seating and taking for granted it will work top opt.

Happy Boating.

 

[yama33]

Re: Prop for 20hp Yamaha

hmm I have a non prog tiny tach perhaps its adding rpms? boat is heavy for a 20 with two batteries, bow mount and fuel gear etc so surprised that i can get full wot with stock prop.

maybe I'll get a prog tiny tach so I know its accurate reading off one wire.

transom is 20in and the yammy I think is 22.2in so cav plate is definitely below boat.

so I should definitely fix motor height before playing with props?

thanks

 

[steelespike]

Re: Prop for 20hp Yamaha

Actually your numbers are right on. Look up performance reports Yamaha.
3 boats, 1,14 ft 833 lbs test weight 12" prop, rpm 5700,speed 26.7 plane 7.3.
2, 1085 test weight,10" prop,23.8 6000 5.20 plane.
3, 1090 10"prop,5900,22.4 6.23 to plane.
I think unfortunately your suffering with 4 stroke hole shot.
Have you tried trimming down a notch with a load?
There is a pretty good chance raising the motor may help
2 1/2" is a lot to be dragging an long.You may not have to drill a hole just raise it up a hole with the same bolts.
I'm probably dreaming.
A 4 blade, 1" less pitch may help but may lose a little top end.
If one battery is just to crank the motor swap it out for a good garden tractor battery.
If the batteries are in the stern move them to the mid point of the boat.Not installed just to see how it does.
Same thing with the gas.
If you don't boat too far a 3 gallon tank may work.
The 20 will run about an hour on 2 gallons of gas at wide open throttle.

 

[yama33]

Re: Prop for 20hp Yamaha

Actually your numbers are right on. Look up performance reports Yamaha.
3 boats, 1,14 ft 833 lbs test weight 12" prop, rpm 5700,speed 26.7 plane 7.3.
2, 1085 test weight,10" prop,23.8 6000 5.20 plane.
3, 1090 10"prop,5900,22.4 6.23 to plane.
I think unfortunately your suffering with 4 stroke hole shot.
Have you tried trimming down a notch with a load?
There is a pretty good chance raising the motor may help
2 1/2" is a lot to be dragging an long.You may not have to drill a hole just raise it up a hole with the same bolts.
I'm probably dreaming.
A 4 blade, 1" less pitch may help but may lose a little top end.
If one battery is just to crank the motor swap it out for a good garden tractor battery.
If the batteries are in the stern move them to the mid point of the boat.Not installed just to see how it does.
Same thing with the gas.
If you don't boat too far a 3 gallon tank may work.
The 20 will run about an hour on 2 gallons of gas at wide open throttle.

just checked the bulletins that's interesting. played with the trim not much different. only have 1/2 inch to move motor up now- yamahas bolts arent strong enough still need to clamp it. so I will need to raise the transom. I posted in the general section lookin for ideas.

I have 2 24 series batteries maybe I can go smaller for the starter/ fish finder battery. One at Stern one at bow currently

just have a 3gal tank in there now

I do notice if the 2nd person moves to very front it planes quicker.

 

[steelespike]

Re: Prop for 20hp Yamaha

No doubt weight placement both temporary and permanent will affect hole shot.
The 2 stroke does have an advantage in hole shot.
We had a 12 foot flat bottom steel boat with a 5 hp gale. With a 180 pounder in the middle seat
it would plane in about 2 boat lengths. 2 or 3 seconds.
Something like smart tabs should help but apply the least pressure to accomplish what you want because too aggressive
use will plow and handle strangely.

 

[yama33]

Re: Prop for 20hp Yamaha

raised motor 2 inches not much difference in performance and rpms still around 6000.

will play with weight distribution to see if that helps.

 

[Sea Rider]

Re: Prop for 20hp Yamaha

You only need to raise engine accordingly if having water splashes over transom, if water flow at plane passes slightly under small upper deflector plate you're ok. If you are already at 6K which is the max rpm stated for that engine, the only thing left would be to distribute well all your gear on deck to balance boat properly. With current load, engine could benefit going one or two pitch more to drop max rpm and better boat's hole shot.

Happy Boating

 

[steelespike]

Re: Prop for 20hp Yamaha

I think you may have miss spoke.If he increases his pitch it will slow his hole shot.
According to the Yamaha tests his numbers are right on.
It appears to me that if weight placement or reduction doesn't solve his hole shot tabs or or a 4 blade is the next stop.
By the way yama33 don't forget a tiller extension so you can pilot from the middle seat.
I saw a youtube of a 10ft jon boat with a 2 hp and a light driver in the middle seat it actually planed.

 

[yama33]

Re: Prop for 20hp Yamaha

I think you may have miss spoke.If he increases his pitch it will slow his hole shot.
According to the Yamaha tests his numbers are right on.
It appears to me that if weight placement or reduction doesn't solve his hole shot tabs or or a 4 blade is the next stop.
By the way yama33 don't forget a tiller extension so you can pilot from the middle seat.
I saw a youtube of a 10ft jon boat with a 2 hp and a light driver in the middle seat it actually planed.

I might consider a 4 blade, what pitch would work based on the numbers?

in the meantime will see if moving battery fwd helps. What about a hydrofoil?

 

[steelespike]

Re: Prop for 20hp Yamaha

They usually suggest a 1" reduction in pitch going to a 4 blade.This should maintain rpm.
If you want to try a foil I would suggest a Doelfin. I can say personally that I had good results on a 3.0 I/O and a 50hp
outboard. Not sure about on a 20 hp,the foil can steal a little top end on any setup and if too deep in the water can cause weird handling.

 

[yama33]

Re: Prop for 20hp Yamaha

They usually suggest a 1" reduction in pitch going to a 4 blade.This should maintain rpm.
If you want to try a foil I would suggest a Doelfin. I can say personally that I had good results on a 3.0 I/O and a 50hp
outboard. Not sure about on a 20 hp,the foil can steal a little top end on any setup and if too deep in the water can cause weird handling.

im about 3/4 inch above the bottom of the boat likely can't go higher with the motor wo cavitation. will see what weight does and then maybe try a 4b

 

[Sea Rider]

Re: Prop for 20hp Yamaha

Hi Steelspike,

Just for the record, if Yama 33 with current load and standard 3 blade-prop pitch is already at max 6K wot ; if going for a less pitch, he should be over reving beyong 6 K, right ? if going for 2more pitch, engine should theoretically decrease his 6K to let say 5600, wouldn't this new rpm help better hole shot ? The other way around, if less pitch helps get out of the hole, when at plane theoretically he should be way over reving 6K right ?

Happy Boating

 

[steelespike]

Re: Prop for 20hp Yamaha

Reducing pitch with a 4 blade is to compensate for the drag and increase in area with the extra blade.
For the best performance with a load the 4 stroke needs to be propped at or near its max rpm in this case I believe 6000.
He's at 6000 with one adult,5700 with 2 adults(one large and one small) and won't plane with 2 large adults.
Rpm should be roughly the same with the reduced pitch 4 blade. The 4 blade design should help it get out of the hole.
It is said to give stern lift on hole shot and bow lift at speed.

 

[yama33]

Re: Prop for 20hp Yamaha

I read in a different thread that the yammy might be slow out of the hold due to the 2.08 ratio. apparently mercs have a 2.15? Would that make much difference given similar displacement? on flipside would yammy have slightly faster top end?

 

[steelespike]

Re: Prop for 20hp Yamaha

I think they would use roughly the same pitch the 2.15 may have a little edge in hole shot.
You could say the 2.08 would have an edge for top speed but would be slightly harder to reach wot rpm with the higher gear.
In the real world you might have to go with a slightly lower pitch with the 2.08 for all around performance.That would eliminate any top speed edge.I do think the motors would be really close in prop selection.

 

[yama33]

Re: Prop for 20hp Yamaha

I wanted to give an update. since raising the motor to just above the bottom of the boat I didn't notice much difference. I added a hydrofoil and it has helped. haven't really lost much top end.

it porpoises a bit at high trim settings so was wondering if I should try and raise the motor just a bit more?

still around max rpm with 1 person so going to 4 blade 1 pitch lower seems to be only option. If I get the weight right might even need to raise pitch given I'm right at 6000

or should I maybe consider moving the 2nd battery to the front of the boat? still think I'm dealing with a heavy stern.

thanks

 

[steelespike]

Re: Prop for 20hp Yamaha

If there is no ventilation you may be able to raise it some more.
Another thought; you could buy or make wedges so you could add or subtract 1/2 a notch of trim.

 

[yama33]

Re: Prop for 20hp Yamaha

had the boat out with 2 people (1 large 1 small adult) and with the hydrofoil it doesn't help near as much as with just me in the boat. still hit around 5700rpm but slow to plane. should I go with a 4 blade 1 or 2 less pitch provided I have 2 people in boat? thinking I still may have issues droppong just 1 pitch based on 2 large adults in the boat.

thanks

 

[steelespike]

Re: Prop for 20hp Yamaha

A 2" reduction in pitch with the 4 blade could add 200 rpm lightly loaded. But surely should help hole shot along with the 4 blade.
You'll need to be aware about rpm.
Once you establish the numbers with the 4 blade try it without the foil.Don't worry about the bolt holes.