Ultra Light Displacement Powerboats

[Willallison]

There seems an almost endless supply of information available for those interested in ULDB (ultra light displacement) sailboats, but virtually nothing about ULDB powerboats. Mike Peters (briefly) refers to them on his web site (mpyd.com) but that's all I've managed to find.
Any suggestions?

[Stephen Ditmore]

If you go to the software forum and click on the various free or low cost software links, there's one of them that has photos of a long, light Phil Bolger design under construction. Here in the New York area Dave Gerr has done some long, light powerboats, and correspondance between him and former Ft Lauderdale, FL mayor Bob Cox on the subject has been published in Southern Boating magazine. Then there's Andy Mele's book on pollution from outboards, which advocates long & light. In high speed boats Fabio Buzzi's might qualify, and among megayachts Martin Francis designs tend to be longer and lighter than others.

[Evolution Yacht]

If you build ultralite make sure you are designed for ultralite. A boat that floats above the designed waterline may have stability problems.

[Willallison]

Many thanks to you both:
Stephen: I have looked at as many of the web sites that you refer to as I can access. Unfortunately the boats depicted in them tend to be either too small or too big! Perhaps I should have given more detail about my ideas.
The concept I have in mind is for a skiff shaped vessel of approx 36ft LOA, to be powered by the latest generation of large 2 or 4-stroke outboards. I am currently exploring the use of aluminium as the hull (and probably superstructure) material for cost and ease of construction reasons, but haven't ruled out the use of composites. A similar concept appears to have been employed by Wally Yachts with their new range of power boats - though on a much grander scale!
Anyway, I appreciate your input - anything else you (or anyone else) can suggest would be greatly appreciated

Will

[twalker]

Sorry that I found this thread late, but I was looking for the boat in Stephen's post... I believe this is it http://www.carlsondesign.com/sneakesy.html - the Phil Bolger Sneakeasy at 27' long and 4' wide - what an absolutely beautiful boat!

Couldn't agree more - she is a great looking boat, though somewhat different to what I had in mind (see my 2nd post above)

[Stephen Ditmore]

Hi Will.

When you say "skiff like" do you mean something like
http://www.rumerys.com/Frost.html ?

You might also want to check out
http://www.zurnyachts.com/

[tom28571]

I'm not exactly certain what you mean by "Ultra light" but I have designed and built a cruising power boat that may fit that description. A powerboat that is "too light" will have it's own set of problems in handling and comfort but performance efficiency can surely be increased over those that are being commercially built. Ruel parker has a 36' power sharpie that may also qualify.

My boat is a 24' Pilothouse Cruiser with a fully fitted hull weight of 1850 lbs and a cruising displacement with engine, two crew, fuel, water and stores of about 2850 lbs. It makes a top speed of 23mph and will hold plane down to 10mph. Power is a single Yamaha T50 four stroke outboard. She gets 8 1/2 mpg in average conditions. The photo shows her running level at 13mph. She stays nearly level at all speeds with the stern never squating. Aft monohedron deadrise is 10 degrees with chine flats turning to a fairly sharp convex bow shape. She is constructed monocoque fashion in stitch & glue plywood. I have named the design The Bluejacket 24 and the prototype is named "Liz" after my wife.

It would be interesting to see what a 30 or 36 foot cruiser of similar characteristics would be like. That is bigger than I have given any thought to.

[Willallison]

Stephen: The rather lovely looking boats at zurn yachts are rather more traditional in hull design than I am considering, and whilst their variation on the Frost is interesting, it wouldn't provide the levels of accomodation that I require.
Tom28571: Love your boat! It's styling is quite different to that which I am working on, but the basic concept is similar.

Unfortunately I not not able to make attachments, otherwise I'd provide a drawing for the kind of thing I'm talking about, but here are some of the preliminary specs:
LOA 11m
LWL 10.5m
BWL 3.5m (max beam occurs at or near transom)
Deadrise aft 8 degrees
Displacement is yet to be determined but I am hoping around 3500kg
Power: 1 or 2 225hp dfi 2-stoke, or 4-stroke outboards

Imagine a hull with a near vertical stem and a very fine entry, gradually widening and flattening towards the transom. The styling is somewhere between traditional and modern, with walkarond decks, a semi enclosed wheelhouse / saloon and permanent accomodation and wc etc in the bow. With a relatively low target weight the boat should perform well with either one (cruise speed in the 20K region and very good economy) or two outboards (higher cruise at some cost to economy)
As far as the hull itself goes, the most similar that I have come across can be found at www.wally.com/WALYYPower/

[Willallison]

Tom28571: One more thought. Don't if you'd be prepared to, but if you could provide me with further details of your boat, I'd be most appreciative.

[Willallison]

Well aren't I the fool. After a quick note from Jeff (thanks) I discover that I can in fact make attachments, so I have done a (very) rough sketch of the type of thing I've been (not very succesfully) trying to describe....
Any thoughts, suggestions, links to vessels of similar types - most welcome

[tom28571]

Will,

I am only an amateur at hull design but my studies make me skeptical of a very sharp entry and a very broad stern, especially in a shallow draft vessel. It could give problems if you need to slow up downwind with the broad stern being caught by a following wave and slewing the boat around with the sharp bow acting as a pivot. Some have met ultimate disaster (capsize) in that scenario.

It is a sleek design though. I happen to like the classic designs of the early 20th century at this time of life. In my salad days, speed was more important.

here are the Bluejacket 24 specifications


Length over all = 24 ft 3 inches = 291 inches

Beam = 96 inches (hull), 98 inches (incl. rubrails)

Beam, waterline (max) = 78 inches (hull), 80 inches ( incl. splashrail)

Beam, waterline (transom) = 70 inches (hull), 72 inches (incl. splashrail)

Headroom (over bunks) = 36 inches

Headroom (forecabin) = 48 inches

Headroom (pilothouse) = 75 inches

Weight, dry w/o engine = 1850 lb

Positive floatation = >1600 lb (foam)

Power = 50hp outboard

Displacement, cruising w/engine, fuel, water, 2 crew & stores = 2850 lb

Freeboard, forward = 48 inches

Freeboard, aft = 34 inches

Speed, max = 23mph

Speed, cruising = 11 to 17 mph

Hope this is of use to you.

Tom Lathrop
Oriental, North Carolina

[Willallison]

I must confess that I too have some concerns about how the boat would perform in a following sea. However, I hope that the underwater sections, as well as a couple of other ideas I have would assist in preventig broaching.

[Stephen Ditmore]

We're getting at some very interesting issues here, and I have more to say than I have time to say it at the moment. I think Tom raises an important point, but the problem is not insurmountable. The way I would quantify it is to look at center of lateral area (CLR), including rudder or running gear, relative to the center of the waterplane (LCF). For good directional stability, it's my experience that CLR should be aft of LCF. This can be achieved with a skeg or fins aft.

We're talking about semi-displacement speeds here, and it's my contention that this is an area where there's still room for improvement in the current state of the art. Slender generally beats wide at these speeds, so multihulls will have lower drag. But lets assume we want to do a monohull for whatever reason.

Will, I think you have the right idea (though you might want to narrow her up a little), but recent research is causing me to question whether an abrupt change from deep to flat might have advantages over a gradual change in terms of wave cancellation.

Here's a 24 foot jet launch I'm working on to illustrate the concept.

I'm looking for a client for this hull shape, and while I can defray some costs if I can maket a kit based on my design (whether 24 or 36 feet), I'm looking to be paid something. With that in mind, any chace you'd consider letting me do the final design of your hull?

[Willallison]

Stephen:
An interesting concept: I'd like to be in a position to explore it further with you. Unfortuantely, I am in fact a student with the Westlawn Inst. of Marine Technology (1st year) and my own project is only a design concept (at this stage anyway - it's unlikely that I'll be building it - in the near future at least).

Your ideas are vaguely reminiscent of a vessel I saw some years ago. It was a commercial boat, approx 65ft long, and was designed to carry passengers out to our Barrier Reef for dive expeditions. Now I come to think of it, it was in some ways a cross between your ideas and mine. It had an extremely fine entry, (the forefoot was actually deeper than the rest of the hull) and carried quite deep sections through till about the last 1/3 of the hull, where it abruptly altered shape to have completey flat sections aft. I'll see if I can dig up some photo's of it that I took at the time and post them. I seem to recall that it was designed in Eastern Europe somewhere and a similar thing was mentioned briefly in a recent Motor Boat & Yachting magazine.

Back to the task at hand though. you refer to operating at semi-displacement speeds, (I am hoping to come up with something that will have a wide range of successful operating speeds) isn't this the sort of speed at which a water jet is least efficient?

[tom28571]

Will, I'm not great at moving images around and even less so at B&W drawings but let's see if this works.

[ Admin Edit: image on remote server seems to have gone offline unfortunately ]

I see that it is a bit weak, but maybe you can make it out.

Tom

[Willallison]

Tom,
I have to say that you did a much better job of posting your drawing than I did!!

[Scott]

If you attach a .gif or .jpg format image it will post inline like the above.

If you attach a .bmp, .zip or other file format, it will post as a link since not all browsers can display these.

It is usually pretty easy to save as either a .gif or .jpg from an image editing program when you go to Save As - to save as a .gif you will have to reduce the number of colors to 256 or less (a paletted image) and 16 or 32 colors is usually sufficient.

P.S. I also really like your boat Tom, and I think your project is very interesting too Will. And definitely Stepen's as well. I'm glad you got this discussion started... 3 great projects - what a great thread!

[Stephen Ditmore]

The answer to the inquiry about the jet drive is yes, I'm taking an efficiancy hit. I think some jets would be better than others, but I've yet to find one that's really right for my purpose, and the Legend Jet is so much cheaper than the larger alternatives that I'm thinking it's cheaper to just throw on some extra horsepower. My reason for a jet is that launches that operate around moorings need to be maneuverable and not get lines tangled in their props. I could do a prop version, but I like the idea of buying the drive as a single unit. Am I completely happy with the solution? No.... but I have a freind who got a slow turning propeller in him when he was young, and had to talk his companion through shutting the motor down without killing him as the blades ripped into his flesh. So I'm inclined to favor jet drives.

[Willallison]

A little explanation goes a long way.....
Actually, looking at your design a little further, the bottom shape is reminiscent of those found on some of the newer displacement catamarans - it may be worthwhile taking a closer look at the shape of their underwater sections.
Do you think that the abrupt shape change will create turbulence in the area of your jets intake?

[Stephen Ditmore]

Optimally the slope of the intake would be the same as the slope of the hull surface just forward of it, so the water would not have to turn up at all, but would simply continue straight into the intake.

The idea here is to create a "pressure bubble" under the stern in the vacinity of the intake. See
http://www.sciam.com/1097issue/1097giles.html
http://www.dt.navy.mil:8765/query.html?qt=Flap&submit=Search

[Willallison]

I'll take a look at those links, but in the meantime, I've managed to track down the photo's of the dive boat that I was talking about. Unfortunately, now that I see them again, they dont't show the aft sections of the hull all that well (I took them about 8yrs ago - on my honeymoon!). But the deep sections of the bottom abruptly stopped and the two shafts ran horizontally out from this. The aft sections of the boat were completely flat. Note also the way that the forefoot was warped downwards giving the hull a distinctly hooked bottom shape. I had my reservations then about problems with bow steering - unfortunately, I never got to see the boat in the water, so I don't know how it performed.