Repower a 43' Planing hull houseboat with diesel sterndrive.

Hi Everyone,
I'm new and my name is Wado. I am restoring a 43' C-Yacht planing Hull style houseboat that is very similar to the Gibson model in this link.
44' Gibson Sport
I am a much better carpenter than engineer and need some help in the propulsion decisions I am trying to make.
The original power was 2 Volvo Penta 260hp engines with sterndrives. I am looking to get the best fuel economy I can and be able to cruise at least 10 knots with one Diesel/Sterndrive package. Maybe one of the Volvo Penta D3 series.
The boat draws about 2' 4" and weighs about 18,500 with 2 gasoline V-8's.

Can anyone assist me in making a good choice of engine/sterndrive to get the best fuel economy and still be able to move the boat at a decent speed?

And can someone explain the term "hull speed" for me? I see it used in posts and am not quite sure what it means.
Thank you all and Happy New Year
Wado

 

Hull speed is typically the 'displacement' speed of a hull, which is 'roughly' the maximum speed your hull will reach in 'efficient' displacement mode before it needs to start planing to go any faster. It is proportional to the waterline length of your hull. At 43ft your max 'efficient hull speed' is probably around 9knts. At 10knts you will be burning a lot more fuel than at 9knts as the boat will be trying to plane, but not actually planing. If you are going to fit smaller engines like the D3's, you probably need to be comfortable with an efficient cruise around 8 - 9knts. Whether you can get on the plane and go faster with good efficiency will depend on how much hp you install. At a rough guess your most 'inefficient' speeds will be between 9 and 13knts.
As for the engines, do yourself a favour and do not buy the D3's. We have 2 of them and they are total garbage, as we have also found Volvo's attitude to supporting them to be. As another comment, the best fuel economy is not through buying the smallest engines. At 8knts my 43ft launch is as efficient with my twin 330hp engines as it would be with twin 150hp engines. If you are buying modern common rail, electronic diesels the larger engines are still pretty efficient at the lower hp's, whereas this might not of always been true with earlier mechanically injected engines where the fuel injection control was harder to optimize.

 

Wow, old thread.

Wado, if you're still around and you consider 9 knots to be a "descent speed", then you are in luck.

A single diesel will do you just fine.

-Tom

 

Aliboy & Submarine! Thank you so much.

I am still around and still working on the restoration. I have not purchased a powerplant as of yet. Many people have urged me to stick with two engines for the additional handling that twins allow but I found a houseboat owner with a very similar boat to mine that has one 130hp gas sterndrive and a sideshift bow thruster. He says that he cruises at 5 to 8 mph very efficiently and docking is a breeze with his setup. So it sounds to me like your analysis is right on.

Do you have any thoughts on the Cummins Mercruiser QSD2.8 220 hp diesel? That is what the local rep recommended for me. ​

Again, thank you for the input. Much appreciated.
Wado

 

220hp is quite a lot for a 2.8l engine, but not at the really silly hp stage. I have owned some CMD 4.2l engines and they have been OK without being really great. I think that the 2.8l is a marinised Isuzu engine or similar. I am never quite convinced about CMD's ability to marinise someone else's engine and make a really good product, whereas Cummins themselves tend to build very good engines. If you could get one I would look at the Yanmar 4LH series engines. They are not the current generation (now the BY marinised BMW engine series) but are one of the most reliable and long lasting small capacity engines in that 200 - 240hp range.

 

Aliboy, I did get a quote on a Yanmar BY series 150hp with one of their ZT-370 sterndrives and it was close to $30,000. I have heard nothing but good about the BY series. I will look into this 4LH series and see what I can find.
Thanks again for the interest and the input.
Wado

 

I know that the earlier 3.0l BY engines in 220 and 240hp had a few issues which are apparently sorted now. A assume that the 150hp is the 1.8l BY engine and purely as a personal observation I don't like these types of small capacity, 'high' hp, engines. The Volvo D3 (which we have 2 of) is similar and whilst I trust Yanmar a lot more than Volvo, people should not confuse these small capacity, high revving, diesels with the slow revving, high torque, long life diesels of previous generations. That 4LH engine for example is approx 3.5l capacity from memory and along with excellent design/quality, the low internal stresses make it a very reliable engine that is simple to maintain. I certainly wouldn't be comfortable in a single engine install with the D3 or BY150 as there is a lot more to go wrong in these significantly more complex engines.

 

Cummins has factory re mans of the very good 6BT series for a very reasonbable price. These range in HP from around 200 to 240 in the mechanical versions and 240+ for the electronic controlled extra valve models. Excellent value for an excellent engine.

Steve

 

Aliboy, Your suggestions make sense to me. I am getting an education. I did look to see if I could find any reman 4LH series engines in my area but have come up empty so far. I will keep searching.

Keysdisease, I will take a look at the 6BT series and see what I can find out. Thanks for the help.

Wado

 

The Cummins 6BT is the same engine as Dodge uses in their Ram Pick up trucks. They are also very popular boat engines and can be found in a variety of commercial applications.

This high volume helps with the up front price, helps a lot with parts pricing and means parts will be available for a very long time. Cummins helps by offering a factory re man product for a great price with a warranty. There are also several companies offering marinizing kits for junkyard Dodge truck motors.

That said I am a big fan of the Yanmar products, but you just don't see them on the used market in the same numbers as other manufacturers.

Steve

 

The international DT 360 or 466 would be my choice.

Tons and tons of them were made for 25 years and are cheap in the wreckers.

Almost any HP , and mechanical or electronic injection, your choice.

FF

 

Is the Cummins 6BT compatible with Mercruiser sterndrives?

Wado

 

Wado - it will depend on what hp you are looking at. The 'trouble' with the 6BT is that it is a 'lower' revving diesel (typically 2600 - 2800rpm max) which means that it makes it's hp via torque rather than rpms (hp = torque x rpms). Sterndrives don't usually have huge capacity for managing high torque, so they are normally mated to high revving engines that max at ~3600rpm+. The gearboxes in the sterndrives won't handle the torque.

Having said that, there are Mercruiser sterndrives now mated to 300hp+ engines, so if you were looking at a 6BT at maybe 150 or 200hp, the latest 'X' series (high torque) drives from Merc may handle the torque quite OK. A quick comparison of the drive spec sheet vs the engine torque & hp curves should tell you what you need to know.

 

aliboy,
sorry for the delay. I had a total knee replacement on Tuesday the 2nd. It is gonna take me awhile to get back to a regular routine.
Now that you mention it, I do recall somebody telling me that fact. My purchase is not going to happen this year. I am hoping to have the boat ready for power next summer. We'll see how the rehab goes.

 

Take it easy on the knee and good luck with the rehab.