Is is feasible to have small inboard diesel in 18-27' centerconsole (sportfisher styl

[Northeaster]

Hi Folks,

Please excuse my ignorance in boat design. I have a 30' sailboat, with an inboard 15hp yanmar (shaft drive). Although I sail 95% of the time, I really enjoy the fuel economy (likely about 1/2 litre / hour) of the inboard diesel. Of course, I only do 6 kts top speed, as the boat weighs near 10,000lbs, and is of course, a displacement hull.

I am looking to build an 18 - 27' center console type powerboat, out of aluminum, and am wondering if there are any designs that use inboard diesels.

I assume that an outboard (say 60 - 125hp) would drive the boat much faster, but in the end, I do not need to do 25 kts ???. (something like 10 - 15 kts) might be nice.
Is its feasible to build such a boat using a small (likely used) diesel with shaft drive (similar to a sailboat)?

Would the boat be able to plane with this type of engine?

If so, what are the implications on engine position, weight , etc?

[michael pierzga]

Sure..you could build a nice 15 knot diesel inboard center console . In Europe where fuel is expensive this type is popular. Several disadvantages. Difficult to handle with a trailer because of the prop shaft, rudder. Interior space is compromised because of installation. On an outbord boat, with its motor hung off the transom, nearly 100 percent of the interior space is free. Cost....a good diesel engine and installation is much more expensive than an outboard.

You could build a very nice 15 knot diesel center console. Id suggest long and thin to move thru the water easily and to best gain interior space. 20 ft ,plus a little, sounds like a good size. Aluminium ? Sure, but the boat will be heavier than a composite boat and more expensive to build

[Katoh]

I am looking to build an 18 - 27' center console type powerboat, out of aluminum, and am wondering if there are any designs that use inboard diesels.

There are plenty of design's on the market, some shaft drive but in that size boat the majority will be I/O. Anything designed with an I/O will work beautifully with a diesel.

I assume that an outboard (say 60 - 125hp) would drive the boat much faster, but in the end, I do not need to do 25 kts ???. (something like 10 - 15 kts) might be nice.

I would be looking at anything from 120-170hp in that size boat.

Would the boat be able to plane with this type of engine?

You bet it will plane, and nicely too.

If so, what are the implications on engine position, weight , etc?

That all depends on design, set-up is it an I/O or shaft.

Ok My boat is 20' driven by a 130hp Volvo Penta I/O Turbo diesel, will do 7-10l/hour at 20-25kn, just drive it as fast or as slow as you want.
Cummins have a smaller 120hp tdi 1.7litre that I'm hearing great things about, used behind the mercruiser leg. This outfit is even being offered in some pressed alloy boats down here in Aus. Very cheap to run and very light compared to mine.
Personally I think its the only way to go for a sport fish boat, Fuels only going to get more exy, and you cant beet the reliability of a good diesel.
Good luck.

[Frosty]

I think you will find that 15kts with 150Hp will be difficult, --not to achieve but maintain.

This speed could be right on the hump where it is nether planing or displacing. This is not a problem but you will have to go at either 6kts or 20 to get any economy --you will not have much in between.

[DCockey]

Originally Posted by Frosty

I think you will find that 15kts with 150Hp will be difficult, --not to achieve but maintain.

This speed could be right on the hump where it is nether planing or displacing. This is not a problem but you will have to go at either 6kts or 20 to get any economy --you will not have much in between.

Depends on the size and hull design. 18 foot to 27 foot is a large range.

We've looked at a 23' lobsterboat which has a displacment of around 3500 lbs and it planes nicely with a 115 HP outboard. It's what some folks would call a "semi-displacment" hull. There is no abrupt hump from displacement speeds to planing. It runs quite happily at 15 knots or so.

[DCockey]

Diesels are not magical. A larger diesel powering a center console boat at 15 knots will use a lot more fuel than the small diesel in your sailboat does at slow speeds.

[liki]

Check e.g. http://www.atkinboatplans.com/Utilit...scueMinor.html and http://www.bateau.com/proddetail.php?prod=ST21 very many older designs will quite closely to your requirements. But if you want to achieve speed with the 15hp engine the boat will need to be light. Or you might be able to achieve semi-displacement speeds lightly loaded and displacement speeds only loaded.

[michael pierzga]

Yes indeed. Google " rescue minor " and observe all the different interpretations of this elegant, proven, true inboard diesel skiff design. Or contract a designer to interpret this design to your needs.

http://www.schleiffboatworks.com/rescue.htm

[Northeaster]

Thanks to all for the great replies. Of course, it makes me realize that I don't even know what I don't know, with respect to boat design.

A couple of thoughts, and clarifications.

I would really like to find a design that I can build in aluminum, as I already weld, and have hopes of building a much larger sail catamaran, in 5 + years time, from Alumium. (looking at the Mumby 48, if you wonder which one!)
Anyway, this small power boat would be a feet wetting in aluminum boat building and the boat would be used by myself, and my father-in-law, for inshore / somewhat offshore fishing on Mahone Bay, Nova Scotia, depending on it's size and stability. I don't have an ideal size as we could have fun on a small runabout, or on a larger, more capable boat that could take us fishing further off-shore, but of course, costs go up for a larger boat and engine.

I really like the look and idea of the Seabright Tunnel 21. However, as the designer's website mentions need to keep the weight low, the difference in chosing alumimum vs wood may be a deal breaker in terms of performance????

- And although I would love the thoughts of an off-shore capable boat, with 140+ hp diesel, this is way, way more money than I want to spend, on the engine, for one. My thoughts are much more along the lines of the Seabright Tunnel 21, or this type of boat, where a reasonably small (sub 30hp perhaps, and likely used) diesel would be able to propel an alumimum hulll at acceptable (for us) speeds.

If anyone has further thoughts on something that may fit this description, all help would be appreciated.

My father-in-law and I have experience welding, fabricating such things as steel trailers, up to 32' long to haul my sailboat, woodsplitters, log haulers, etc, so we are up for the challenge of making systems like engine, steering, cooling, etc work on a boat.

[Easy Rider]

Look at "Tang", a 26' inboard by the Atkins designed to run 15mph on 35hp. It's a wood design but has all straight lines so is a good candidate for aluminum. I want to build this boat but the proper wood construction looks too time consuming. Wish I knew how to weld. I could weld in cold and wet weather and here in Alaska aluminum is king. Very few people here will buy anything else. I'm going to send for the plans. I'd like to use my 60hp outboard on Tang. See the link on liki's post for Atkins boat plans. Then find Tang under cruisers to 27'. Then PM me or possible phone talk. One downside of your idea is that you'll be sitting right on top of the engine.

[viking north]

Northeaster--Alum. verses wood shouldn't be a dealbreaker--Alum' should work out to be lighter. That size boat you'll be working with some 3/16th. to 1/4in. 6000 series framing and 5000 series 1/8 to 3/16 th in. skin. However the weight is the least of your worries, unless you've welded alum. before and have punched your hours of hard learning it would be better for you to pre cut the hull and hire an experienced alum. welder. Therse guys are specialists with alot of hours under the hood using their own or rented top quality gear. I'm an experienced welder including tig. and mig alum. on small projects but shy away from hull work and in the past have hired outside help on the bigger builds.-- I'm just an hour up the coast from you and a quickly winding down boatbuilder building my last vessel . Any questions PM me. ---Geo.

[Northeaster]

Well, one of the guys at Boden Boat Plans was great in answering a few of my questions, and I am currently waiting on my local metal / aluminum supply dealer to price up the materials list for the following design.

http://www.bodenboatplans.com/produc...products_id=24

I takes between a 60 - 210HOP diesel, with 140 being recommended, I believe. I will be keeping my eye out for a used diesel in the 60 - 140 hp range, and waiting to see how expensive the aluminum will be.

Thanks for all of the input. If the materials cost is too high for me, I amy have to look at a smaller design.

[viking north]

As we say on the Rock Narester Me Son--Giv er By -keep us updated including photos on the boatbuilding section of the forum. My build under such-- Building the Nancy G----

[Northeaster]

I was discouraged yesterday, as I got a couple prices, which worked out t $6 - 10 / bl for alumium of various grades. Feeling better today after visiting another supplier. He will cost out materials list in next day or two, but just taking a couple examples of sheet specs needed, he was thinking it would be roughly $2.50 / pound (Cnd $ which equals US$ at the moment).
Even if it averaged $3 / lb, at the 1500 lb hull would be around $5000 in alumium, which IA could certainly live with.

Do those numbers seem roughly approiate? He was looking at 5083 for plate/ sheet and 6061 for framing ,etc.

[Submarine Tom]

So, maybe now is the time to look at used outboard boats for sale.

Hint: Look south of the border. If you fly down to pick it up you can buy a truck and trailer to get you and it back, sell the truck and perhaps even MAKE money on the whole deal...

-Tom