Increasing speed by changing gear ratio, prop size

dnauhei

Senior Member
Joined
Jun 16, 2015
Posts
216
Likes
31
Location
Charleston, SC
Boat Make
'78 Sisu 22, Power: inline 6 cyl diesel Chrysler Nissan SD-33 (aka CN6-33)
Short version: For optimal speed on a 22" full displacement hull, should I do a 14" prop with a 1.1 gear? Or a 15" prop with a 1.51 gear?

EDIT: My prop is a 14 L 12, so not a 15". And I don't think I have space to swing a bigger prop.

Long version:
I have a '79 Sisu 22 with an Nissan Chrysler SD33, a straight 6 with a 15". RPM range is 2600 cruise - 3200 WOT (although the high end of that RPM may have been set lower, but I have had no tach up unitl now).

I did an engine swap for a better condition Sd33 last winter, but it had a 2.67 different gear. My speed dropped by about 30%. So I am changing the transmission ration this winter. I don't know what the old ratio was, but I believe it was 1:51. So . .

With the 1:15 I got 9 kts cruise - 11.5 WOT
With the 2.67 I get 5 kts cruise - 9 WOT (high end of throttle may be set higher)

Others are reporting higher top speeds out of this hull. Another '79 Sisu is getting 9 kts cruise - 16 kts WOt with a 14" prop and 1.1 gear. That with a Perkins 4.236, which is a straight-4, 85 HP. 2000 cruise, 2700 WOT I think. Yet another is reporting 13 kts at WOT with a 65 hp engine.

I would like to get more speed out this hull with my 85 hp engine. Should I try a lower gear ration with a small prop?

Or . . . I have noticed that the cruising speed for the comparison boats is the same as mine with my old gear (9 kts); maybe I am already at the optimal gear vs prop ration and the only reason my WOT speed is not higher is the high end of my throttle was set too low.

And is the fact that the boat can swing a 15" prop say the designed thought that was the correct prop size?

Should I try 1.1 w 14 prop or stick with 1.57 w 15" prop? Or if it ain't broke . . .

Thank you!
-dn

18953037_10155499560159456_2452680840088616430_n.jpg
 
Last edited:

Blitzen

Captain
Joined
Sep 1, 2011
Posts
4,170
Likes
2,774
Location
Sumday Isle, RI
Boat Make
Flowers Boat Works 46, 1991 Boston Whaler Outrage 19
Do you have a boatdiesel.com membership? If not join up ad run your known numbers through the prop calculator and once you get the constants right then insert different gear ratios and see what you come up with. The 25 bucks for a membership will be way worth the knowledge you gain.
 

dnauhei

Senior Member
Joined
Jun 16, 2015
Posts
216
Likes
31
Location
Charleston, SC
Boat Make
'78 Sisu 22, Power: inline 6 cyl diesel Chrysler Nissan SD-33 (aka CN6-33)
Do you have a boatdiesel.com membership? If not join up ad run your known numbers through the prop calculator and once you get the constants right then insert different gear ratios and see what you come up with. The 25 bucks for a membership will be way worth the knowledge you gain.
That's a good thought. I remember that forum although I forgot it was devoted to marine diesels. They are going to ask me for some technical data I don't have. I need an opinion based on experience and intuition, which this forum is full of.
 

eyecatcher

Senior Member
Joined
Sep 30, 2012
Posts
186
Likes
55
Are you looking to race this or something? If they all do 9 knots cruise I think you will be wasting your time. What is the proper loading wot RPM for this engine? Do you know the full wet weight of this boat loaded with fuel persons etc..? For DE, the rule of thumb is you want the largest size prop you can fit with proper clearance and match the gear to that just like the larger boats with proper engine loading. With a semi displacement hull you want to have the slowest shaft rpm and the largest wheel close to square like 15x15 or 14x15 will generally give you the best cruise performance. A 1.1 gear I would think you would have to go to a much smaller prop than a 14" something like a 13 which is a bit small. Its difficult to compare different downeast boats with different engines since they are all a bit custom. If you can fit a 15"wheel pretty sure the 1.5 is the right ratio. . If you were able to fit a larger wheel then the larger ratio's 1.75 or 2:1 would be desirable. If you can only fit the 15 the 2.67 is not the right gear that is why you lost a lot of speed because you probably didn't increase the prop size/pitch.
 
Last edited:

crabz

Senior Member
Joined
Nov 2, 2015
Posts
375
Likes
137
Location
Alaska
Boat Make
All American
Is 15" your max prop diameter?

15" sounds under propped for your 2.67 (?)
 

dnauhei

Senior Member
Joined
Jun 16, 2015
Posts
216
Likes
31
Location
Charleston, SC
Boat Make
'78 Sisu 22, Power: inline 6 cyl diesel Chrysler Nissan SD-33 (aka CN6-33)
Are you looking to race this or something? If they all do 9 knots cruise I think you will be wasting your time. What is the proper loading wot RPM for this engine? Do you know the full wet weight of this boat loaded with fuel persons etc..? For DE, the rule of thumb is you want the largest size prop you can fit with proper clearance and match the gear to that just like the larger boats with proper engine loading. With a semi displacement hull you want to have the slowest shaft rpm and the largest wheel close to square like 15x15 or 14x15 will generally give you the best cruise performance. A 1.1 gear I would think you would have to go to a much smaller prop than a 14" something like a 13 which is a bit small. Its difficult to compare different downeast boats with different engines since they are all a bit custom. If you can fit a 15"wheel pretty sure the 1.5 is the right ratio. . If you were able to fit a larger wheel then the larger ratio's 1.75 or 2:1 would be desirable. If you can only fit the 15 the 2.67 is not the right gear that is why you lost a lot of speed because you probably didn't increase the prop size/pitch.
Not looking to race, just want to get the most speed I reasonably can out of the hull. I was mistaken in the original post, my prop is 14" and I really don't think there is room to swing a bigger prop; there is pic below, let me know if you agree. The only reason it had a 2.67 is that I bought an engine that had been in a 30' wooden lobster boat. I knew the transmission ratio would be wrong for my lighter and smaller 22', but my mechanic said to go with it and see. So I did, and now it is time to adjust. I don't believe this is a semidisplacement hull, but in any event, that is interesting to know. Maybe I should go with the 1.51, which I believe is the correct ratio, and try a more "square" wheel. What would that mean, is there such a thing as 14 L 14?

83946949_10158125599459456_8803606779729543168_o.jpg
 

Kailua Kid

Senior Member
Joined
Jan 14, 2019
Posts
229
Likes
127
Location
Bainbridge Island
Website
www.gibbonslawgroup.com
Boat Make
General Marine 26; Klamath 19; Sparkman & Stephens Yankee 38 sloop
Not looking to race, just want to get the most speed I reasonably can out of the hull. I was mistaken in the original post, my prop is 14" and I really don't think there is room to swing a bigger prop; there is pic below, let me know if you agree. The only reason it had a 2.67 is that I bought an engine that had been in a 30' wooden lobster boat. I knew the transmission ratio would be wrong for my lighter and smaller 22', but my mechanic said to go with it and see. So I did, and now it is time to adjust. I don't believe this is a semidisplacement hull, but in any event, that is interesting to know. Maybe I should go with the 1.51, which I believe is the correct ratio, and try a more "square" wheel. What would that mean, is there such a thing as 14 L 14?
That looks like the largest wheel you can have on that boat, maybe even a tad bit over sized. There is a formula for calculating prop tip clearance that you can find with an internet search. From what you describe, you should consider changing your gearing back to or close to where you were before the engine swap. My guess (and I am certainly not an expert) is that you cannot get the speed back by just re-pitching that wheel.
 

eyecatcher

Senior Member
Joined
Sep 30, 2012
Posts
186
Likes
55
Pretty sure you should hit at least 3800-4000 rpm wide open with that engine. You can't compare it to a Perkins. You should photo tach your rpm first to determine the load with the current prop. You dont want to overload the engine, you might be at least 600rpm low. Based on the info you provided and your engine a 1.5 would be the optimum ratio. You should drop that in and figure out what the rpms are first with the prop you have then adjust the prop from there with better data. Otherwise you would be guessing. Even the calculators wont get it right.

It looks like 14 is about the max diameter. Your prop should be about right on with a 1.5 gear. Once you figure it out maybe 4 blade 13X11 be the right prop but you need to figure out what your doing first with the equipment you have. If it all works right you should be around 16-18 knots at 3800-4000 wot
 

dnauhei

Senior Member
Joined
Jun 16, 2015
Posts
216
Likes
31
Location
Charleston, SC
Boat Make
'78 Sisu 22, Power: inline 6 cyl diesel Chrysler Nissan SD-33 (aka CN6-33)
Pretty sure you should hit at least 3800-4000 rpm wide open with that engine. You can't compare it to a Perkins. You should photo tach your rpm first to determine the load with the current prop. You dont want to overload the engine, you might be at least 600rpm low. Based on the info you provided and your engine a 1.5 would be the optimum ratio. You should drop that in and figure out what the rpms are first with the prop you have then adjust the prop from there with better data. Otherwise you would be guessing. Even the calculators wont get it right.

It looks like 14 is about the max diameter. Your prop should be about right on with a 1.5 gear. Once you figure it out maybe 4 blade 13X11 be the right prop but you need to figure out what your doing first with the equipment you have. If it all works right you should be around 16-18 knots at 3800-4000 wot
This is the performance curve from the manual, and it only goes up to 3200. I take that as meaning that as high as the engine should be run. Am I over interpreting that?

20190604_102618.jpg
 

eyecatcher

Senior Member
Joined
Sep 30, 2012
Posts
186
Likes
55
So your engine is not rated for 85 hp? looks like that is showing continuous and intermittent duty. Possibly yours is detuned or industrial. I could be wrong. Since you have the manual. Do you have the specs for max rated output? Most all had a tag on the block. AFAIK all non turbo were 85hp 3800 and the later models 94hp 3600. Turbos were 4000 rpm. What is the bare idle no load rpm ? If it is in the 4000+ rpm range then it is def not a 3200rpm engine.
 
Last edited:

dnauhei

Senior Member
Joined
Jun 16, 2015
Posts
216
Likes
31
Location
Charleston, SC
Boat Make
'78 Sisu 22, Power: inline 6 cyl diesel Chrysler Nissan SD-33 (aka CN6-33)
The manual states "maximum output: (3200 rmp) 73 hp. So clearly I was wrong about the HP.

I don't know what bare idle no load is rpm is. There is nothing listed for that. Maybe that's the low end of this range: 1200 rpp, 30 hp?

I don't know if this is helpful by maximum torque is (1,600 rpm) 133 ft lb.

IMG_4719.JPG
 

dnauhei

Senior Member
Joined
Jun 16, 2015
Posts
216
Likes
31
Location
Charleston, SC
Boat Make
'78 Sisu 22, Power: inline 6 cyl diesel Chrysler Nissan SD-33 (aka CN6-33)
So your engine is not rated for 85 hp? looks like that is showing continuous and intermittent duty. Possibly yours is detuned or industrial. I could be wrong. Since you have the manual. Do you have the specs for max rated output? Most all had a tag on the block. AFAIK all non turbo were 85hp 3800 and the later models 94hp 3600. Turbos were 4000 rpm. What is the bare idle no load rpm ? If it is in the 4000+ rpm range then it is def not a 3200rpm engine.
Let me correct this. It may be that "Rated output" is 3200 (72 Hp). In the manual it calls this "maximum output." The pic I posted is actually the faceplat from my old enginge, so the new one could say something different. But it is an hours drive, so I can't check right now. Elsewhere I have read that max output on this egnine it 3800 and 83 hp. Like I said, old engine, hard to find reliable info. Part of the problem is that there is no uniformity in the terminology used. If someone asks, what HP is your engine, I have no idea how to answer that.
 

Brooksie

Captain
Joined
Mar 25, 2013
Posts
1,326
Likes
465
Location
Cape Cod
Boat Make
Bruno-Stillman 35
I think you can believe the tag on the actual engine you are using. Usually 1200-1400 shaft rpm works best on downeast hulls. So 2:1 would get you in the ballpark. 3 blade props are usually faster albeit with more vibration than 4 blades but 4 blades seem to cruise and carry loads more efficiently w/ less vibration.
And yes you need 15% of prop diameter clearance over the prop and 10% under.
You should be able to get more speed if you now have 83hp.
A 22' heavy wooden downeast I helped build planes out and does 9.5K with 50 hp and the wrong gear ratio.
 
Last edited:

eyecatcher

Senior Member
Joined
Sep 30, 2012
Posts
186
Likes
55
Bare idle no load rpm is the fuel stop or mechanical governor fuel limit on the pump when the boat is in neutral(not in gear) you should be able to turn up to about 3500rpm. The plate is the number to go on for when you are loaded. You want to hit 3200 rpm at wot or just over with the biggest prop you can fit with proper clearance. From that you can reverse calculate the best gear ratio to use. Just want to protect your engine. When it is setup right those engines are good for over 10,000 hours.

Like brooksie advised the 2:1 is ideal for downeasts, but you don't have enough clearance to run a 17 or 18"prop. This is common. Something between 1.2-1.5 is the ratio that will work the best for your setup with a 13 or 14" prop.

A lower gear and prop logic sometimes works better on a planing hull for top speed but too much keel in the way blocking water flow for a high shaft speed. It will just have more prop slip and cavitation being less efficient but you may go just as fast at the end of the day. Hard to say until you try it. I'd say your prop is about right on for your setup with a 1.2-1.5:1 gear in front of it and work it from there. If all you have a 1:1 then that will work but you'll most likely have to drop the prop size. Depending on how heavy your boat is, it may be a wash. Hope that helps.
 
Last edited:

Staff online

Latest Posts

Top