Transmission ratio and props

[Toolsntoys]

I have decided to bit the bullet and wrote the check for new engines. I am installing Cummins qsc 550s in my 53 motor yacht. Currently the transmission are 2-1 with 28x31 props The cumming dealer is recommending going with 2.5-1 with 28x33 props. Is there any advantage to slower turning props with a bigger bite then faster turning with less bite. The new engines max rpm is 3000 and I’m guessing cruise rpm would be around 2400. (80%). It still like to have the boat run efficiently at 9knot though. Thoughts ? I know of two other 53 that have been repowered. Pascal and John lines. Both have kept the 2-1 ratio Pascal has 28x24. And John has 28x27.

[Boatsb]

Qsm11.

The qsc is only 8.3 liters and is just to small.

If your spending the money do it right. It will work out better in the end.


Shaft and prop size is determined by many things. Reduction makes it turn slower and puts more stress per revolution. At 3000 rpm and 550 hp you need to follow the dealers guidance if he has done the math.

Few people here have done these repowers and of the ones that did the jury's probably still out on a few. If the dealer is to be trusted stop second guessing him with armchair mechanics.

[jim rosenthal]

One of our cruising club members did this repower with earlier C series cummins, at 480hp, and was happy with the results. (John Lines' boat, Freebird) I think the difference comes down to the fact that less top-end is expected from MYs, so what would be not adequate in a convertible works fine in a MY. If the dealer thinks you should use a deeper gear, he's probably right. The only caution there is that with the deeper gear, you can swing bigger wheels- but they have to fit under the boat with adequate clearance. I think overall you will get more efficient drive with larger, slower-turning props than with smaller wheels turning faster. If you think of propeller slip as a percentage of driving efficiency, the faster the wheel turns, the more overall thrust is lost to slip. Keeping wheel rpms low and getting your thrust from more blade area lessens your losses to slip.

[Pascal]

There is a limit to the amount of money you can sanely pour in such a project. What s the price difference with the QSM?

These are not sportfish where it s all about bragging rights... an extra 100hp over the C Series is plenty.

A good prop shop can run the number and confirm the best package. A second professional opinion is always a good idea

[Toolsntoys]

I’m very confident I made the right choice going with these engines The qsm11 is also a great choice but just not for me I plan on cruising the inter coastal at 9 knots with an occasional 18 My Cummins dealer is very knowledgeable and does a lot of repowers. I am trying to better understand his though process when suggesting what he did while others have gone a different route.

[jim rosenthal]

I think you made the right choice as well. I don't think you would use the power from M11s, although they are very good engines and absolutely the ticket for a lot of convertibles. As far as the deeper gear and bigger wheel at slow rpms, I think he is also right. But you DO need enough room under there to make it all work as it's intended to do.

The weight reduction also factors into the performance and economy improvements as well- you probably save a thousand pounds per side with the new engines and gears, all of which weight less. But what will REALLY be lighter is your wallet. Like full of helium.....

[Boatsb]

Your dropping a couple of liters of displacement from the original 8v71. Also I've seen QSM11 engines at 330 Hp for long range cruising.

The engine mass and displacement make a big difference.

[Pearl Island]

I own since May 2015 a 45c 1990 re-powered in 2007 with QSM11 660Hp.
The engines had 1140 hours when I bought the boat and now have 2200hrs.

I am very happy with the QSM11, it is a very reliable engine.
Maintenance cost is low as long you do not run it hard. So far the engine have never failed and only maintenance have been oil, filter, zinc and a impeller change. The belt also need to be tighten about every 200hrs.

Boatdiesel.com is a excellent place to find information and if you look for exhaust manifold leak you will find this to be a critical point on the QSM11. It seems like the root cause is that it was originally a truck engine that was converted to a marine engine keeping the dry exhaust and turbos. The consensus seems to be, running the engines hard and you can have a cracked exhaust manifold at as low as 500hrs.. Limit your max fuel consumption to 17gls/hr/engine 1,800-1,900 rpm (~350hp) and it will last for ever. For better accuracy, install EGT probes and keep an eye on it.
As for any engine it is important that the boat is not overpropped and you get max rpm at WOT, otherwise the engines will run hotter.
Considering this, you will probably be fine with the qsc 550s unless you want to get more power out of the QSM11 and spend a fortune in maintenance.

[SKYCHENEY]

What shaft diameter are they specifying for the 2.5 to 1 gears? You might be able to stick with 2" shafts if you don't go with deeper gear reduction. This could be a huge savings as you don't have to change struts.

[Toolsntoys]

Sky. He is telling me that the 2” shafts will be fine with the 2.5-1 ratio. It does seem that more prop pitch would put more stress on the shafts