Repower twin prop with single engine

The friendliest place on the web for anyone who enjoys boating.
If you have answers, please help by responding to the unanswered posts.

fcdrifter13

Newbie
Joined
Apr 12, 2024
Messages
1
Location
Myrtle Beach
Hey all first post, been doing some learning.

Have a chance to buy a 43ft GulfStar MKII that has sadly had both of its perkins 120hp engines destroyed both to water ingestion. The boat is in otherwise great condition. I also have access to a Cat 4.4 t4f engine that has been dpf, scr, and def deleted. This engine at 100% load uses 8.1gph and produces 250hp@2200rpm and 600ft/lbs@1400rpm.

Had the idea pop into my head to throw two Linde 55 hydrostat motors on the back through a twindisk mount and run the props hydraulically. While the total overall HP would decrease the torque is increased by 200ftlbs. There would be a considerable weight savings as well as space savings so I could put in larger fuel tanks if wanted. Was only looking to cruise at 5.5-7knts not looking to get anywhere in a hurry. From my understanding the boat currently has 22 x 17 props.

Would someone be able to break down the math on what i would be looking at power consumption wise with those props. I found a few calculators that all give me different answers. One says that the power required would only be 21hp to go 7knts and that does not seem right as I could deliver that power at just off idle to both props and even may be able to increase speed through the hydrostat with the available torque.
 
These boats sell in the 30-50K range with good power. It's hard to imagine it would be worth it to repower one unless it dropped in your lap for free and you had a line on replacement engines for <10K. Sorry to be a Debbie downer on this but I don't see much upside to this deal. If the boat were in pristine conditions and you find some good take-outs then maybe, but you will almost sure to be under water. Changing running gear from twin to single or doing something more elaborate such as trying to create a twin from a single via hydraulics as you suggest would be very expensive and make it even more unattractive financially.
 
A few thoughts:

Any time you convert power from one form to another, there will be loss or reduced efficiency. So, converting engine (mechanical) power to hydraulic power will be a loss. Converting hydraulic power to mechanical power will also be a loss. Essentially, a direct drive through a transmission will be significantly more efficient.

The Gulfstars were supposed to have fairly efficient hulls. My 45' boat has a semi displacement hull which isn't quite as efficient. To push it at 7 knots requires 40 HP. 6 knots is 24 HP. These numbers are based on a fairly accurate consumption of fuel multiplied by 20 HP per gallon consumed. While the Gulfstar hull may be more efficient than my boat, the conversion from mechanical to hydraulic and back again will have a significant penalty.

It sounds like an interesting exercise, but from a long term cost standpoint, fuel consumption may offset replacing the drive trains with similar ones.

Ted
 
First I think your ideas are financially irresponsible. I understand there is no requirement to be financially responsible in this world so if you are board and this sounds like fun have at it.

21 hp to push 43’ at 7kts sounds a bit optimistic but I would guess you are close theoretically. In the real world you have wind, waves and dirty bottoms. This all requires HP to over come so are we asking 7kts in perfect conditions or 7kts in the real world. Also note, the power curve is exponential so while it’s easy to make the 7kts it won’t be easy to make 8kts.
 

Attachments

  • 46137-76aec983b1e79642ce325813c6de6687.jpg
    46137-76aec983b1e79642ce325813c6de6687.jpg
    10.5 KB · Views: 34
  • 46138-c0424e87e9b8a98ba7e6f620b7f48bac.jpg
    46138-c0424e87e9b8a98ba7e6f620b7f48bac.jpg
    12 KB · Views: 34
Last edited:
Fun project that makes absolutely no financial sense to do. My guess is that costs would quickly spiral to heights never considered.
Repower costs have gotten crazy expensive. I wonder how many boats end up in the dumpster simply because it makes no sense to spend the money to repower them?
 
Fun project that makes absolutely no financial sense to do. My guess is that costs would quickly spiral to heights never considered.
Repower costs have gotten crazy expensive. I wonder how many boats end up in the dumpster simply because it makes no sense to spend the money to repower them?
Lots and lots!
 
I know of one boat that was powered by diesel / hydraulic drive. It was sort of a success. When it worked. It was not very reliable. One of the problems was cooling the hydraulic fluid.
 
I know of one boat that was powered by diesel / hydraulic drive. It was sort of a success. When it worked. It was not very reliable. One of the problems was cooling the hydraulic fluid.
Cooling the hydraulic fluid is indeed a major design consideration. Far more heat will be generated than a transmission. Heat generation is a large part of the power losses as mentioned by Ted.

I’d guess the best route would be to rebuild the current engines unless the boat was a sinker. Those old Perkins 4s are quite rebuildable.
 
Thinking about it, if someone really wanted to do a setup like this, diesel/electric might be more practical (and more efficient) than diesel/hydraulic. But I don't think it makes much sense either way, it's probably better to either rebuild the engines or find a pair or suitable replacements to install.

Twin 120s is probably a lot more power than that boat needs, so something like the 85hp Betas might be a viable repower option if the Perkins are too far gone for a rebuild.
 
I've been mulling this over for a couple of days ( I clearly need to get a life). One thought was if it would be more practical and efficient to make a spilt drive off a single engine using timing belts and a custom assembly to drive two transmissions offset on either side of the single.
This project would involve a bit of engineering to make the splitter, but I think with the low hp you would be just barely in the range of a XXH Gates belt. The splitter contraption/mount would attach to a modified bellhousing on the main and would contain the shafts and timing pulleys to offset the shafts to roughly the spacing of the shafts. Then two transmissions like the Hurth ZF 45A would be mounted to the aft portion of the splitter. Jackshafts would then couple the output of the transmissions to the shafts via an Aqua Drive which would provide the thrust load support.
You would need to sort out the mounting, and clearing the stringers might be an issue, and you would need the ability to shift the engine forward about 4' or so which would likely be a no-go on most boats.

Totally impractical of course, but in theory I think it could work and if it stayed together would be 98% efficient.
 
I once toyed with the idea of converting a twin engine (Perkins 165s) Mainship Pilot 40 into a single engine. I had it pretty well planned out, it would have been either a C series or B series Cummins and a single screw.
But I bought a 40 Albin instead.
 
Thinking about it, if someone really wanted to do a setup like this, diesel/electric might be more practical (and more efficient) than diesel/hydraulic.
There was an article in Passage Maker maybe 15 years ago on converting twins to a single generator with twin electric motors to drive the shafts. I believe Siemens supplied the electric motors and they used a Cummins 6BT to drive a 500+ volt generator. My recollection was that it was successful but not more efficient. As I recall, the electric motors were water cooled in some way. Generating heat other that in the diesel engine is a sure sign of lost efficiency.

Ted
 
There was an article in Passage Maker maybe 15 years ago on converting twins to a single generator with twin electric motors to drive the shafts. I believe Siemens supplied the electric motors and they used a Cummins 6BT to drive a 500+ volt generator. My recollection was that it was successful but not more efficient. As I recall, the electric motors were water cooled in some way. Generating heat other that in the diesel engine is a sure sign of lost efficiency.

Ted
I wouldn't expect it to be more efficient than straight diesel drive, but almost certainly better than hydraulic. Although on a faster boat diesel/electric might make more sense. 2 engines, 1 big, 1 small, so each is optimized efficiency wise for running fast vs slow. I'm not sure if you could gain enough engine efficiency for slow running to offset the higher drive losses though.
 
Hey all first post, been doing some learning.

Have a chance to buy a 43ft GulfStar MKII that has sadly had both of its perkins 120hp engines destroyed both to water ingestion. The boat is in otherwise great condition. I also have access to a Cat 4.4 t4f engine that has been dpf, scr, and def deleted. This engine at 100% load uses 8.1gph and produces 250hp@2200rpm and 600ft/lbs@1400rpm.

Had the idea pop into my head to throw two Linde 55 hydrostat motors on the back through a twindisk mount and run the props hydraulically. While the total overall HP would decrease the torque is increased by 200ftlbs. There would be a considerable weight savings as well as space savings so I could put in larger fuel tanks if wanted. Was only looking to cruise at 5.5-7knts not looking to get anywhere in a hurry. From my understanding the boat currently has 22 x 17 props.

Would someone be able to break down the math on what i would be looking at power consumption wise with those props. I found a few calculators that all give me different answers. One says that the power required would only be 21hp to go 7knts and that does not seem right as I could deliver that power at just off idle to both props and even may be able to increase speed through the hydrostat with the available torque.
Wow...an incredible amount of work would be needed to convert everything (rudders, struts, etc.) from twin to single. If I were in your boat (pun intended) I might be looking at repowering with electric motors from Electric Yacht in Minnesota or other alternatives.
 
You'd have to be able to send variable power; one prop more than the other in turning and maneuvering situations, does the boat have a bow thruster? The ability to turn in tight quarters using just the motors is one of the benefits of twins so seems like you'd want that. Also you will have the dual morse controls up in the pilot house (and fly bridge if it has one) so those would need to be either connected to the new setup, or replaced...
 
If you have the skill set to do it yourself (mostly) and you like that stuff. Great. I DIYed the re-power of my first boat and the peripherals were the same cost as the brand new motor. And that was a drop in. I can only imagine the peripherals for this to be 2 - 4 X. But as I said...
 
Back
Top Bottom