Help with Honda BF 20 HP prop on 16ft wooden dory

[spottydog]

I have a 16ft flat bottom fibreglass over ply dory. It has a console steer and false floor. I am not sure what it weighs to be honest.

What I do know is the the prop that came with the BF20D motor, a 4 blade 9.25 x 10", is not getting the motor near it 5000-6000 rpm range.

I have tested it with 2 people and 1 person with different trim settings on the motor. I can only achieve 5000 rpm WOT. Measured by a Teleflex tacho which runs off the alternator.

The motor feels/sounds like it is lugging also.

I spoke to a Solas prop rep who said I should use a 3 blade prop on my rig. He suggested a 3 blade Amita 3 9.25 x 8".

My thinking was that is a huge step, I guess my next testing will tell. If the motor revs over 6000rpm at WOT I will try a 9" pitch Amita 3.

The rep said the Amita 3 has a large surface area, more than some 3 blade designs.

Does all this sound like I am heading the right direction?

I will post the new results after this weekend.

Thanks in advance.

 

[spottydog]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

The speed measured on my GPS is;

With 2 people I get about 15mph, with 1 person 21mph.

 

[jestor68]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

That prop change should net you about 400-600 rpm increase; 100-200 rpm going from 4 to 3 blades and 300-400 rpm dropping 2 inches of pitch.

Simply switching to a 3 blade of the same pitch(10) should net a gain of 100-200 rpm and result in more top speed. If a 9 pitch is available, I might try that in 3 blade first.

Your boat probably has a dry weight(boat/motor) of 400-500 lbs.

 

[steelespike]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

Does the boat have a nice broad bottom with a straight run aft so t can get up on the water and level out.
If it has rocker in the bottom it may try to squat.Also if the bottom narrows toward the stern it my squat.
Is the bottom nice and smooth like a factory bottom?

 

[spottydog]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

Does the boat have a nice broad bottom with a straight run aft so t can get up on the water and level out.
If it has rocker in the bottom it may try to squat.Also if the bottom narrows toward the stern it my squat.
Is the bottom nice and smooth like a factory bottom?

Here is a pic of the bottom (before top coat paint). Its flat, dead flat. 4ft wide til it hits the chine. Finish is gloss.

 

[steelespike]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

Other than being a little narrow it should help get the boat up on the water.3 keels may slow it slightly but should help it track well
and stay on course at no wake speed.
The 6 mph difference adding one person seems like a lot. I wonder if it puts more chine in the water adding more drag.
Being so close to the bottom rpm doesn't help either possibly exacerbating the speed loss and adding drag.
No doubt shooting for closer to the 6000 rpm will help hole shot and load ability and general performance but may limit top end a
little.It will be interesting to see what the 8" does for your rpm.

 

[steelespike]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

Other than being a little narrow it should help get the boat up on the water.3 keels may slow it slightly but should help it track well
and stay on course at no wake speed.
The 6 mph difference adding one person seems like a lot. I wonder if it puts more chine in the water adding more drag.
Being so close to the bottom rpm doesn't help either possibly exacerbating the speed loss and adding drag.
No doubt shooting for closer to the 6000 rpm will help hole shot and load ability and general performance but may limit top end a
little.It will be interesting to see what the 8" does for your rpm.
Check if the anti vent plate just above the prop is about even with the bottom of the boat.
Sorry wrong button.

 

[spottydog]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

Other than being a little narrow it should help get the boat up on the water.3 keels may slow it slightly but should help it track well
and stay on course at no wake speed.
The 6 mph difference adding one person seems like a lot. I wonder if it puts more chine in the water adding more drag.
Being so close to the bottom rpm doesn't help either possibly exacerbating the speed loss and adding drag.
No doubt shooting for closer to the 6000 rpm will help hole shot and load ability and general performance but may limit top end a
little.It will be interesting to see what the 8" does for your rpm.
Check if the anti vent plate just above the prop is about even with the bottom of the boat.
Sorry wrong button.

Anti vent plate is a little below the bottom. Maybe up to 1". Safe side.

If the 8" provides to much rpm I will try 9". It is an interesting exercise. Appreciate the input.

 

[steelespike]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

It's possible you could raise the motor as much as 2" and still have good all around performance and a little additional speed.
The less motor dragging through the water the better.
Depending on the angle the boat planes at the antivent plate could be 2 or more inches under water.
The plate should be just visable at speed.

 

[spottydog]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

Tried the 3 blade 8" prop. Developed too much RPM. Not sure how much exactly because the enigne must have hit a rev limiter then the tacho floated around between 4-5k RPM (so it must have been getting false pulses or something?)

I could only just manage 15mph with one person no load as well for all that reving.

I have sent it back to try a 3 blade 9" pitch later this week.

 

[steelespike]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

So your saying a 2" reduction added about 1000 rpm.
It is my feeling that on a smaller motor that may be the case.
The rpm change per inch is higher on smaller motors.
So it appears a 9" will get you about 5500 rpm.
Will be interesting to see the final; rpm and speed.

 

[steelespike]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

So your saying a 2" reduction added about 1000 rpm.
It is my feeling that on a smaller motor that may be the case.
The rpm change per inch is higher on smaller motors.
So it appears a 9" will get you about 5500 rpm.
Will be interesting to see the final; rpm and speed.
Your slip at 5000 rpm and 21 mph was about 8%
a 9" prop at 5500 and 8% slip nets about 20.7 mph.
The rpm should hold up better with the 9.

 

[spottydog]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

So your saying a 2" reduction added about 1000 rpm.
It is my feeling that on a smaller motor that may be the case.
The rpm change per inch is higher on smaller motors.
So it appears a 9" will get you about 5500 rpm.
Will be interesting to see the final; rpm and speed.
Your slip at 5000 rpm and 21 mph was about 8%
a 9" prop at 5500 and 8% slip nets about 20.7 mph.
The rpm should hold up better with the 9.

I also changed from a 4 blade to 3 which was a big change.

I am going to try both a 4 and 3 blade 9".

 

[spottydog]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

I also changed from a 4 blade to 3 which was a big change.

I am going to try both a 4 and 3 blade 9".

4 blade 9.25x9 got it in the right area. Big improvement on the factory 9.25x10". Top end gained maybe 200rpm and 2mph (around 24mph). Big difference was better speed across the range. She will sit nicely at about 4.5k rpm for about 15mph.

Next is the 3 blade 9.25 x9"

 

[steelespike]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

Something isn't quite right.Can't go 23 at 5200. So either the rpm is reading low or your speed is reading high.I'll do some more figuring.

 

[steelespike]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

Something isn't quite right.Can't go 23 at 5200. So either the rpm is reading low or your speed is reading high.
Assuming 8% slip like the original prop 23 mph would be about 6100 rpm.
Was the speed by gps?Originally you quoted 10" prop 5000 rpm at 21 mph I figured 8% slip.
I think you have a miss print above.
You said gained 2 mph and quoted 24 mph result. Shouldn't that be 23, based on your original 21 mph?

 

[spottydog]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

I was wondering about my tacho reading. Everything there is set up right however.

I do have a laser tacho on the way to test things out.

I am having to convert kms/ph to mph also.

Its doing 38km/hr at 5200rpm. Motor even sounds better, like its not lugging.

 

[spottydog]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

I was wondering about my tacho reading. Everything there is set up right however.

I do have a laser tacho on the way to test things out.

I am having to convert kms/ph to mph also.

Its doing 38km/hr at 5200rpm. Motor even sounds better, like its not lugging.

Speeds are off the GPS. SO there is some inaccuracy depending on what direction I run, against or with the tide. I will try this prop out more but I already sounds and feels better than the 10" 4 blade prop.

 

[steelespike]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

It surely is better, I bet it feels peppier.
I guess you could make a 2 way run and average it , or would you run across the tide?
Or perhaps drift in the tide to get its speed.
The site I use says 38 km is 23.61 mph, assuming the same 8% slip as the 10" indicates rpm is 6231.
With the increased rpm the slip might be lower which would lower the prop calulator rpm.
It would appear that a 9" 3 blade will further increase the rpm.
Is it a Honda Tach?
I assume the max rpm for the 20 is 6,000.

 

[spottydog]

Re: Help with Honda BF 20 HP prop on 16ft wooden dory

It surely is better, I bet it feels peppier.
I guess you could make a 2 way run and average it , or would you run across the tide?
Or perhaps drift in the tide to get its speed.
The site I use says 38 km is 23.61 mph, assuming the same 8% slip as the 10" indicates rpm is 6231.
With the increased rpm the slip might be lower which would lower the prop calulator rpm.
It would appear that a 9" 3 blade will further increase the rpm.
Is it a Honda Tach?
I assume the max rpm for the 20 is 6,000.

Tacho is TELEFLEX LIDO 7000 RPM W/ hour meter. I have it set to 6pulses which is what is should be for the motor. No doubt there.

Max for 20 is 6000rpm. I read somewhere 20hp is developed at 5500rpm, but that was just someone else s post.

Will have to wait a week maybe before i test the 3 blade.