Gear Ratio question

Russ

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Hi Folks,
Quick question - I'm currently running a TAMD41B with a MS4B gear at 1.96:1 ratio in a Cape Dory 28. I've got a line on a HS1 gear at a great price which I sorely need. The HS1 has a ratio of 2:62 to 1. Can I make up this change with the prop? Which direction would I have to go? Diameter , pitch etc. Thanks for any help
Best Regards,
Russ
 

Powderpro

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Hi Folks,
Quick question - I'm currently running a TAMD41B with a MS4B gear at 1.96:1 ratio in a Cape Dory 28. I've got a line on a HS1 gear at a great price which I sorely need. The HS1 has a ratio of 2:62 to 1. Can I make up this change with the prop? Which direction would I have to go? Diameter , pitch etc. Thanks for any help
Best Regards,
Russ
Yes you can make it up with the prop, ideally you would go with the largest diameter prop that will comfortably fit under the boat, and adjust prop pitch to load the engine. You want to be able to hit the engine’s maximum rated rpms with a full load (fuel, water, gear, crew) on the boat.
 

leaky

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I don't understand the science behind it but there are some ideal speeds to be turning a shaft at, limitations in the size of the prop that go along with any boat, and limitations to how far you can go on pitch.

One piece of information missing here is the max RPM for that engine & current pitch/diameter, maybe also what size you can fit.

With the max RPM, current pitch, you would know a ballpark of what you would need in pitch to makeup for the change in gear ratio.
 

Genius

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Putting your information into a prop calculator can help you with finding solutions you need. Do one for what you have now, then manipulate it with the proposed gear change. Boatdiesel dot com has one that I use regularly.
 

bigjay

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Hi Russ,

I feel your pain, I own a CD 28 FB and agonized over repowering or replacing the MS4 transmission.
Unfortunately, I never found a good tranny to drop in.
The previous owner had put time and money into the 41 engine which was reliable but smoky, but the transmission started causing issues for us.
The HS1a may have clearance issues in the CD bulkhead and require cutting as I remember. if you are able or have some good marine mechanical friends, maybe try a z gear.
Cheaper, newer, better.
The cape dory 28 site is gone along with a archive of good information.
so the 2.62 is reducing output rpm, after that I am going to defer to other folks here.
But my guess is adjust pitch, state whether you have the stock 3 blade or the 4 blade that will help the old salts here guide you.
That being said you are in new territory, you can try marine parts express in maine for some insight, I believe they have rebuilt the HS1 for a CD owner

We replaced our 41 with a yanmar with a BW drive. Very happy with performance.
Feel free to PM, these classics are worth keeping
Cheers
 

Russ

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Thanks guys,
WOT under load should be 3800. The vessel was a project when I got it and just this fall was the first chance I was able to do some real sea trials. Currently has a high quality Nibral 4 blade. 20 x 21. With great boost pressure, clean bottom and engine running well I'm only turning 3400. while leaking tranny fluid and terrible grinding while shifting, I know it's not going to be as smooth as a hydraulic gear but this is beyond the noise it should make. ( hard to find neutral as well) I believe most of these Volvo equipped CD28's came from the factory with a 3 blade 18 x18. I've talked to the previous owner and he said that is what the prop shop recommended. It's clearly over propped right now. Am I correct in the thought that the 2.62 gear ratio might increase WOT rpm with the current prop?
I was hoping the HS1A would require less work to swap out than the ZF. I can get this HS1 for a grand.
I appreciate all the feedback.

I just love this hull. I can't walk off the dock without turning around, and admiring her lines.

CD28_WRC.JPG
 

leaky

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Thanks guys,
WOT under load should be 3800. The vessel was a project when I got it and just this fall was the first chance I was able to do some real sea trials. Currently has a high quality Nibral 4 blade. 20 x 21. With great boost pressure, clean bottom and engine running well I'm only turning 3400. while leaking tranny fluid and terrible grinding while shifting, I know it's not going to be as smooth as a hydraulic gear but this is beyond the noise it should make. ( hard to find neutral as well) I believe most of these Volvo equipped CD28's came from the factory with a 3 blade 18 x18. I've talked to the previous owner and he said that is what the prop shop recommended. It's clearly over propped right now. Am I correct in the thought that the 2.62 gear ratio might increase WOT rpm with the current prop?
I was hoping the HS1A would require less work to swap out than the ZF. I can get this HS1 for a grand.
I appreciate all the feedback.

I just love this hull. I can't walk off the dock without turning around, and admiring her lines.

View attachment 125526

Prop pitch is the number of inches a prop would theoretically move forward in one rotation. It's very similar to putting bigger or smaller tires on your truck - except a boat has a fixed gear ratio (ie it ain't a 6 speed)...

If you go to a 2.6 ratio with the same prop, you have geared down your engine and absolutely at any given speed you RPM goes up, as does wide open throttle.

2.0 to 2.6 is a 30% reduction. Your prop pitch then likely needs to go up very similarly, to around a 26 or 27 pitch - ballpark. Depending what happens with that which changes prop slip, if you go up in diameter, add cup, a 3 blade, etc etc, maybe you end up with a 25 or 28. But something like that anyway.
 

Powderpro

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If you are currently only able to get 3,400rpm with 2:1, the 2.6:1 reduction should allow your engine to reach its target of 3,800 rpm. I would see how your current prop works with the new transmission before thinking about tweaking the prop. You will have a lot more information to go off once you swap transmissions and do a sea trial. Don’t waste your time and money trying to guess what prop might work before testing your current prop.
 

leaky

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Add - I missed the part about RPM being wrong at WOT today, was really saying to be at 3400 RPM that's a ballpark. So as stated - prop is gonna be a lot closer to right than 6 pitch off.
 

Russ

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Thanks for the valuable insight everyone Sounds like the HS1 with the geared down ratio is doable. Any other negatives to the 2.6 ratio I should be aware of? I'd rather be under propped than over. Using my limited math skills it looks like it may be very close with the current prop. Maybe slightly under propped which is better than over. Thanks again.
 

ccc

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Think about shaft diameter also. Usually not TOO big a concern in a rec boat, but if yours is old or marginally sized to begin with, it would at least be nice to know. You will increase torque as you decrease shaft rpm.
 

bigjay

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To Confirm, the stock wheel for the CD 28 with the 41a was 18x18
I have a 4 blade 19x17 and was able to reach 3800 rpm with the 41a
stock shaft is 1.5 dia

Russ, I have a box of spare parts for the 41a, you are welcome to them gratis
If you need engine parts not available anymore, try French Creek Marina in upstate NY. they sell used parts and have 41a components used.
I also have good documentation on parts replaced, so I can give cross references to things like injectors etc. Have CD manual and engine manual too.
Just sing out if you need anything or have a CD question.
BTW check your steering ram, if it is a seastar with a ball post pivot, you should replace it. They were on the earlier 28's and were updated due to the post snapping under load. I replaced my hydraulics when refitting and found out then.
Keep an eye on gelcoat crazing especially around the cockpit, mostly cosmetic but we had some that needed attention.
We filled and faired, then put down 3 coats of barrier coat followed by Awlgrip. Like new IMG_3689.JPG



Yes they are a handsome, well built downeaster, we love ours IMG_2190.JPG
 

Russ

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Big Jay - That cockpit looks sweet. I know to well what it probably looked like before you started. Thanks for the offer of spare parts. I'm in eastern NC. Where are you located?
 

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