Considering Repowering Again

It’s important to keep in mind that repowering an old DD with a modern diesel will give you a rejuvenated boat. In this case, lighter, faster more fuel efficient and more engine information underway. And hopefully quieter. That is the real best case or upside, you will have a better product and you get the satisfaction of a refit well done.

Modern props will go a long ways towards making a better quality of ride , less vibration and more responsive Plan on replacing your shafts , cutlass bearings and maybe shaft seals at the same time.

In a previous thread on YF you had 2-1/2” shafts, because of the old school 1.6/1 gear ratio. You may have to go from a 4 blade to 5 blade prop with the new engine choice, but all that can be calculated and optimized prior to starting the project.

1.97/1 is definitely a better situation for 1150 hp and 2300rpms. Sounds like you should be good on shafts, already saving money

My rules for a repower - First , reduce weight and gain a bit more horsepower, as your cruise speed will definitely increase and you will get better fuel economy. Second - get new propos, as there have been gains in prop design resulting in better performance since your boat was built - Veem or Michigan Wheel.

Check your gear ratio - I looked back in the Donzi archives and when you bought the boat you stated "The gears are Detriot 272 A with a 1.6:1 ratio. I think these were made by Allison for DD." This would explain the 2,5" Shaft diameter because a 1.97/1 ratio would require at least a 2-3/4" Shaft for the 12V-92TA's. Check it out again to make sure.

The 2.5" Shafts do not have the right Safety Factor for the CAT C-18's rated at 1150hp if you are using a 2.0/1 gear ratio, even with AQ 22 High-Strength Shaft material. For recreational Diesel boats you need at least a calculated safety factor of 5.0. You would have to go to a minimum of 2-3/4 Shaft, AQ 22 HS. But keep in mind that a 2.0/1 gear ratio gives you a better prop, i.e., less potential cavitation as it slows it down a bit. Ideally, you would design the propulsion system around a perfect gear ratio, a compromise of space available due to shaft angle and a low incidence of predicted cavitation. This is what gives you the best cruise at the smoothest levels of vibration. A CAT C-32 definitely requires 3.0" shafts, assuming you are going with a rating in the 1450 - 1600 hp range.

The air intakes for a C-18 should be fine in your cooler Alaskan climate. Higher power C-32's would have to be calculated and may (would) require a change. The C-32's are just too much weight in my opinion, and the MTU 10V2000 would be a better choice if higher speeds are your goal.

C-18's and a SeaKeeper, if you don't already have one, look like the best bang for the buck IMO.

If you are keeping the Shafts at 2.5" Diameter, then you will have to remain in the 1100hp / 2300 rpm / 1.6:1 Gear Ratio range. Twin Disc had some gear models supplied by Niigata, a Japanese company. Will have to assume the shaft material is AQ 22 High strength.

This would eliminate the C-32, so the focus would be on a C-18 which gives you a better weight savings anyways, especially if you have no need for the SeaKeeper.

So the next focus would be on propeller selection, as 1.6/1 ratio is fairly challenging, your blade tip speeds are typically considered high, you'll have to watch cavitation inception and should start that discussion with the propeller suppliers in your area. You can manage some of it by going to a 5-Blade design, Veem or Michigan Wheel. They can provide the calcs, especially if you have a ballpark weight estimate to go with your current cruise/wot speeds.

I would stick with C18's as you'll see about 3 knots more speed than the Detroits at 86 GPH @ 80% load for both. The C32's, aside from being much physically larger and much heavier will be fuel hogs in the boat, 1600 HP C32's will burn about 125 GPH @ 80% (both) and probably only another 4 knots of speed over C 18's, plus the expense of new shafts/struts/ etc.

I wouldn't touch Michigan Wheels in a SF, they are poor technology in comparison to Veems and with this gear ratio you will absolutely get prop burn with the shaft speeds With Michigans. Veems would be my choice. You can also go with a splined shaft/prop (instead of keyways) and that will also increase your safety margin on the same size shaft, if you're replacing shafts. Most custom SF run splined shafts/props and thinner shafts to increase speed/efficiency. Also I wouldn't be in such a hurry to switch to 5 blades either, unless you need the stern lift, as you'll get more speed with 4 blades. I just ran a 62' MY with 1200 mans, and 4 blade Veems and I believe it had 2" shafts and around a 2.0:1 gear ratio, cruised at 26 knots +/-.

If a 1.6:1 gear ratio has worked fine in the boat, the boat will be faster than going to a 2:1 gear ratio. The faster you spin a prop, (lowest reduction gear you can get away with) the faster the boat runs usually (speed) as you get a little more lift. I worked on a 2003, 75' Custom SF, full tuna tower, twin 2000 HP engines, 2.5" shafts, 1.5:1 reduction gear, 4 blade props, and it cruised 35 knots, topped out at 43 knots. It performed perfectly. At 1200 rpms the boat was fully on plane and did 20 knots.

I have tested and used pretty much all of the current prop manufacturers out there. I really like Veem for their manufacturing quality and quietness underway, when you receive them fresh from the factory they are like a piece of jewelry, absolutely flawless.. With that being said, the Michigan Wheel M-500 props are as technically sound as the Veem's. They have been designed by one of the leading hydrodynamic experts in the country and are a bit more affordable than the Veem's. All propellers have experienced burn at some time or another, the successful ones manage it better than the others. The Hatteras GT 60 had a problem and went back and forth between Veem and MW and ended up with MW I believe. Viking has partnered with Veem and has remained steady with them.

Splined shafts do not reduce the shaft diameter or increase the safety factor, as the shaft size are calculated with Torsional Strength as the main material criteria. The gear ratio drives the shaft diameter for a given horsepower and the Torsional Strength of the shaft material, and the old school 1.5 (1.6) /1 ratios are not seen as often now as the hp increases have put further demand on the propellers. What we have seen is an increase in Propeller blade area, number of blades, and better blade design, especially at the blade root/intersection of the propeller hub. Not all burn issues are attributed solely to propellers, as the strut design, hull strakes and thru-hulls can be problematic if they affect the inflow to the propeller in a adverse way.

Comparing custom SF or MY's data is an Apples v Oranges conversation. You can easily look at production SF models and you would be hard pressed to find an old school 1.5/1 gear ratio amongst them, look at the Hatt 59, Viking 58, Bertram 61. With the weight of a production boats and the higher hp on new engines, you wont see these gear ratios. The fastest production SF boats today all have gear ratio's over 2.0/1. Comparing prop speed on gear ratio alone completely ignores efficiency/slip/cavitation/vibration, all the stuff that really matters when engineering a correct propulsion system.

For clarification, Michigan Wheel has a lot of propellers under their brand. The premium SF wheel would be the Marlin Series and even the CX series under their Federal propeller line: https://www.miwheel.com/propellers/brands/federal-propellers/

Great, looking forward to the repower. Have you settle on a gear ratio yet?

Yes, but their metallurgy isn't anywhere near Veems, and they don't perform as good as Veems.

True on the metallurgy, Veem uses a higher amount of virgin copper than others due to being near the source in Australia.

Not so true on performance, just keep tabs on the top dog sf, , not necessarily the production guys.

I've used both on several brand new SF over the years, on the same boats with both propellors, the Veems aside from having less vibration, were around 1 knot faster. On one boat the builder had to take one set of Michigans off at 48 hours because they were so prop burned it looks like they'd fling a blade and put Veems on it and all was well in the world. On high HP fast SF, the Michigans will flex.

That’s a smart upgrade to go with the 665 gear, nice to have the peace of mind for the better duty cycle. The 1,525 gear ratio will keep the 2.5” shafts in good shape with a safety factor over 5.

You can take the original prop and get them cleaned up and reconditioned to use as a test prop on sea trials and gather the performance data. Investing in the repower, it makes sense to finish it off with a new prop once you know the numbers.