Matching Gear Reduction to Engines and to Props

[GoodChance]

Anyone here good with matching gear reductions to engines and props?

Looking at a New Yanmar 315 and a 2.5 gear. I think taht the gear is too large of a reduction for a Holland since I will only be able to swing a 20" (Diameter) wheel. I realize that the pitch is the bigger driving factor here but what are your thoughts on a 2.5 reduction on a 32 Holland and a relativel small skeg/keel and prop?

 

[fishinwishin]

I sometimes dabbel but without even thinking about it. 2.5 is too much reduction.

 

[fishinwishin]

I'll guess about a 20 x 32 to maybe 34 wheel with at idle speed about 7 mph. min.

 

[GoodChance]

I was thinking that a 20x28 wheel would match a 315 Yanmar to a 2.5:1 reduction gear but I am just guessing.

The "salesman" says that a 2.5 gear would work just fine ...... but this is what he has in stock and wants to sell it. I am highly doubtful though. Like to hear other thoughts though.

 

[fishinwishin]

I was thinking that a 20x28 wheel would match a 315 Yanmar to a 2.5:1 reduction gear but I am just guessing.

The "salesman" says that a 2.5 gear would work just fine ...... but this is what he has in stock and wants to sell it. I am highly doubtful though. Like to hear other thoughts though.

I probably should have asked rather than assume the RPM is 3000 on one of those yanmars.. What is the rated RPM. If its any higher than 3000. The wheel would be less pitch as you are thinking.

 

[GoodChance]

I probably should have asked rather than assume the RPM is 3000 on one of those yanmars.. What is the rated RPM. If its any higher than 3000. The wheel would be less pitch as you are thinking.

3800 is the max rpm on the 315 Yanmar. Pretty high. And normally I'd never consider such a righ reving engine but this particular engine has a great reputation, has been successfully used in a number of DE hulls including the 28 BHM (a pretty hard boat to push), the engine is local to me and the price is very good.

 

[fishinwishin]

Oh wow. I didn't think the RPM is way up there. I'd think 20x 28 might be pushing it. You might be down to 20 x 26. 5mph idle speed and 32 wot

 

[GoodChance]

Oh wow. I didn't think the RPM is way up there. I'd think 20x 28 might be pushing it. You might be down to 20 x 26. 5mph idle speed and 32 wot

Just spoke to a Yanmar rep ...... he's telling me taht the higher rpm engines need the 2.5 reductions to be equilivent to the 1.75 and 2:1 reductions of a traditional 2800 rpm engine. And the more numbers I run with pencil to paper, I think he is 100% correct.

BillD .... when you get back here, run over to BoatDiesel and run these numbers through their calculator. Figure a hull weight of 10,000 to 11,000lbs.

Ronnie Raider - You have a 315 Yanmar in your 28 BHM, righ? What's your gear ratio? And your prop size?

 

[GoodChance]

Oh wow. I didn't think the RPM is way up there. I'd think 20x 28 might be pushing it. You might be down to 20 x 26. 5mph idle speed and 32 wot

I think that I would be running at 3000 rpm, with the prop spinning at 1200 rpm through a 2.5 reduction and giving me about 20 kts with a 23p wheel and 10-12% slip.

On my old Cat 3208 I was turning the engine at 2200rpm in a 2.0 reduction giving a prop speed of 1100 rpm.

Again, the 2.5 reduction is probably spot-on.

 

[captainlarry84]

2.5 yes!!

The 315 Yanmar @ 3800 RPMs work excellent with the 2.5 gear in New England boats. With a 2.5 you would swing a 22 X 22 wheel. However you can only swing a 20 so your propeller would be 20 X 24. If you dropped to a 2:0 you gain 25% shaft speed and your wheel size would decrease to a 20 X 18.
I would go with the 2:5 gear and larger wheel and a 1 shaft a must. With the 2.5 you have a lot more room to fine tune the wheel with cup & pitch as apposed to the smaller wheel.
If you are doing a total repower on this boat and reshafting is required. Do it right and push in a wet tube. I bet if you pushed the tube back a little and faired the bottom some you could get that 22 X 22 in there and you would be golden!

 

[BillD]

Call Larry @ Accutech Propeller or H&H Propeller Salem, Ma.

Chris, on Monday, call Larry @ Accutech Propeller Dover, NH. He & and his son run the business. A very large Class I prop shop.

Also call H&H Propeller in Salem Ma.

Between these two shops they do most of the prop work for the Maine DE builders and keep a database of "real performance numbers" on the boats the've propped.

Larry had a bunch of numbers on Calvins, H&H's, Flowers etc... builds I looked up for "powderpro".

I'll mess with the prop calculator on boatdiesel, but I'm not very experienced with it LOL!

 

[captainlarry84]

agreed on the PROS!

I concur with BillD playing with propellers is very expensive. Plus the last thing you want to do is re-pitching. The less you bend them the better. Propellers are casted in different pitches so it is nice to get one casted to exactly what you need.
The rule of thumb on re pitching is 1” for Nibral & 2” for Bronze. You can go more but it is not a good idea.

Those shops should get you within the 1’ range for sure. I re did your wheels size in a 20” blade and based on my math which is a backyard data base of know information & some Kentucky Windage I compute your Propeller to be a 20 X 22 with the 2.5 gear. I would love to see how my numbers compare with the pros.

With a 20 X 22 you could also stay with a 1 ½ “shaft. Going larger is not always good. The max min range for a 20” propeller hub is 1 3/8” to 1 ¾” so 1 ½” put you in the middle. In addition there is extra turning weight & drag that will cost you shaft horsepower.

I would also ask them about shaft sizing & discuss a wet tube it will really make the run smoother and state of the art.

Attached is a photo of my Class I 23 X 21 Nibral Propeller. It has never been re-pitched. I figured this size propeller out using my math when I did my repower. S&S on L.I. Classed the wheel and added a very light #3 cup to the edge. On my 370 Yanmar with a 2:0 gear I get 3400 RPMs some days & 3300 on other days pending the load. At 3100 I get 19 knots & at WOT 22 knots. My propeller clearance is only about 1 ½’ however with the very light cup I get no rubble & she is velvet smooth.

My propeller efficiency is about 78% not bad for a keeled boat.

We must also keep in mind that even though pitch is what pushes your forward. The Diameter is what gives you your efficiency.

If a 22 X 22 has efficiency of 80%

& a 21 X 20 has efficiency of 90 %

The 21 X 20 would be faster than the 22 X 22 even though the pitch is less

90% of 20” = 18” of forward movement per revolution

80% of 22 = 17.60” of forward movement per revolution

The key is to find the most efficient match & on Keel boats that is why diameter is so important as a lot of the wheels efficiency is block by the keel leaving the field of the blade to do the work.

 

[Sailorgp]

You might want to get a sizing from Michigan Wheel...just my 2 cents

Prop-It-Right Sizing Request // Michigan Wheel Marine

 

[captainlarry84]

I second that idea plus bore size.

 

[captainlarry84]

Everything must match!

Bore sizing is as important as the correct wheel. Two little bore and a light shaft, the shaft snaps. To larger bore and the hub gets weak & you throw a blade. In addition two large of a shaft is a lot of turning weigh & water drag directly off the motor as it is just starting to make H.P.