2 engines to 1

[luke scheidecker]

hi this is my first time on here i live in southeast ak i own a 1972 uniflite express cruiser with two crysler 318s i am planning on pulling them and putting a 4 cly volvo in its place iwas wondering if anyone has ever changed from two screws to one and some of the problems they may have encounterd im a marine diesel mecanic so im mostly wondering about putting the shaft out the keel instead of two out the sides would really value some in thanks

 

[oops!]

Re: 2 engines to 1

welcome to iboats !

i have not heard of any one doing that conversion.
there is a lot to that. the shaft angle would be a main consideration, as well as adding a rudder.
thats a lot of extra parts.....do you have a doner boat?

 

[luke scheidecker]

Re: 2 engines to 1

im pretty sure the angel is the same and i was going to use the parts i take off the shafts are 1 inch onboth engines crysler and volvo so i was going to use the tru hull for the shaft of of one of them for the volvo and then take one or two of the struts and move them down i should have two rudders if i pull them out or could build a new one out of stainless and have 3 rudders one screw??????

 

[NoKlu]

Re: 2 engines to 1

Sounds like a huge drop in power. From what 400 down to 140? Not too many guys going backwards like that. You should check the power rating on the hull and see where it's rated at. You might not even get on plane.

 

[oops!]

Re: 2 engines to 1

im going to have to do some investigation on this ...but unless you are using a reverse set up like in the new wake boats.....you will have to move the motor to amid ships.
something your hull was not designed for. there will be other considerations besides just the shaft out puts and rudder.....(i would just go single). your math has to be bang on. as far as weight distribution.

to run a mid ship setup.....you will have to completely renovate your salon area as well as loose precious deck space.......have you drawn the plans?....post your drawings and let us look them over

 

[coastalcruiser]

Re: 2 engines to 1

just throwing this out there,
Obviously your after going hull speed and saving fuel.
i did meet a guy that swapped his 454 for a small diesel and he was raving about the fuel ecomomy
If it were me I would probably consider a stern drive or sell and buy a single engine boat.

 

[drewpster]

Re: 2 engines to 1

I have a single screw inboard. You may have some ballasting issues when you remove the second engine, so that is a consideration. However ballast is easy enough to add.
As far as mounting the setup it would seem to me to be fairly straight forward. You would have to close the old holes in the hull but that is certainly do-able. And you would have to modify the stringer system and fab new mounts. (also do-able). My deciding factor would be how much room you have in the bilge. It would not be worth it if you have to modify the whole deck. Try to research Uniflight and see if they offered the boat you are working on in a single screw. There may be some design differences, but I doubt they tooled two seperate lines to build the boat with different drive setups. It is far more likely they used the same plans for each setup and modified the mounts. There was a Uniflight that berthed at my home marina...nice boat...but I think it was a twin screw as well.

 

[23SX]

Re: 2 engines to 1

I would leave it twin engine, sell it and get a single if that's what you want. That would be a huge project and most people would give up after getting started on something like that but not saying you would.

 

[petermarcus]

Re: 2 engines to 1

I'm actually a little familiar with Uniflites, and they're good boats. Especially in Pacific waters like yours -- they're real workhorses and I've found them to be pretty dry in choppy water. Some years had problems with hull blistering, but these days, if it's still around, it's probably a good boat.

Just off the top of my head, it may be possible to convert a couple 318s to a single diesel, but I'd go for one screw, not some geared double or triple screw arrangement. With the weight of that boat and possible cargo, I'd think the stresses on the linkages would be pretty high, especially in rough seas. If one linkage broke, it might completely cripple the other screw(s). Even if there was some cutout, you're talking about driving the boat in an emergency with forces changing 90 degrees and a few feet off to the side of centerline. With a single shaft, I'd think the forces would be transferred more directly on the axis of the mass of the boat.

The biggest issue would be torque on the prop to make sure you can get and stay on plane, and that basically means horsepower. If you can get similar horsepower out of a single diesel as the two 318s, then it's probably doable -- and for the sake of all of us, please, please open a thread on the conversion so we can all watch!

(Edit: I initially read it as two or three props, not rudders. So ignore that part about the screws.)

 

[oops!]

Re: 2 engines to 1

peter......i dont think it will plane moving it the motor to the center of the hull.

i really think it will end up being a displacement hull........this is due to the change in the center of gravity.
yep......there are a lot of mid engine designs......but they are for totally different hulls.
i think to keep the plaining aspect......he will have to go with a reverse setup like the new wake boats.....then we are also talking about gears to get the low revving diesel to put out the rpm needed to plane a hull

 

[Unleadedaddict]

Re: 2 engines to 1

peter......i dont think it will plane moving it the motor to the center of the hull.

i really think it will end up being a displacement hull........this is due to the change in the center of gravity.
yep......there are a lot of mid engine designs......but they are for totally different hulls.
i think to keep the plaining aspect......he will have to go with a reverse setup like the new wake boats.....then we are also talking about gears to get the low revving diesel to put out the rpm needed to plane a hull

Lets say a diesel turns 200 hp 2000 rpm with 400 ftlbs. Totally do-able numbers correct? With that if you want a 6000 rpm capable
shaft, you can overdrive the gear to 1:3 then in theory it would put 3 times more force on the engine than 1:1 So you would need alot more engine to get the boat going. Torque and Hp would be cut by a third. Not the right way to go IMO.

Now my question to the OP is why do you want go to a single screw? There is a point where you lose economy on a smaller engine vs 2. If you have to keep the little engine wide open, I am sure 2 V8's could achieve better gph at the same cruising speed. It seems like in my mind that it would be a step backwards and alot of money spent without pleasurable results. Now if you wanted to repower with twin diesels IMO seems like a win win decision. Good luck with whatever you do.

 

[petermarcus]

Re: 2 engines to 1

I think Unleadedaddict hit the nail on the head when it comes to "Why?" Other than Luke saying he's a diesel guy, and there's a definite cost difference (do you already have access to a diesel?)

Oops, I don't have a fraction of the hull knowledge you do, but I'm wondering what the center of gravity issue is all about. The Uniflites I've seen have two hatches in the cockpit with the engines below those, and empty space between them. There may be a stringer in there, which would complicate things (I don't remember offhand, but that would definitely be an issue). I'm trying to picture this in 3D roll/pitch/yaw, but it seems to me if there are two (say) 500 pound engines to either side of the fore-aft centerline of the boat, the center of gravity averages to the middle, right on the centerline. If you put a diesel in the center, right on the fore-aft centerline, sure, it's not going to be 1000 pounds, but the center of gravity should be the same in that axis, right? Hypothetically if you ballasted on that centerline with some lead or concrete or something so the same weight was on that centerline, wouldn't the weight or mass or whatever cancel out? Forget the power/torque considerations for now, I'm just curious how the hull would behave.

 

[oops!]

Re: 2 engines to 1

yes...you can offset the balance with weight.....it is commonly done......its the fore aft that is really crutial.

but the lower you can get the weight ....cg....the better for the righting arm

 

[drewpster]

Re: 2 engines to 1

oops, do you have an equation for determining righting moment?

Seems to me it would be a matter of maintaining the same ballast. The greater the righting moment, the lessor the tendency for the boat to heel over...correct? If somebody has the equation we could just input the numbers. Of course we would have to know specific data to input.......

geez, my head hurts....:facepalm:

 

[Insaneplmr]

Re: 2 engines to 1

I'm actually a little familiar with Uniflites, and they're good boats. Especially in Pacific waters like yours -- they're real workhorses and I've found them to be pretty dry in choppy water. Some years had problems with hull blistering, but these days, if it's still around, it's probably a good boat.

I was searching for some info as I am going to look at a 1969 31' uniflite cruiser with the twin 318's, aside from the blistering does anyone know about some weak spots to look for? Any feed back would be greatly appreciated.

As far as changing the drive, It can be done but, time and money willing, but is the price worth the cost. In other words, after spending all that time and money, will the end result be worth the effort. If you do it and it works well let me know, maybe I will have bought this one I am about to look at and do the same