Playing around with a 10 m trailer cruiser

[marshmat]

Thanks for the tips Will
Beautiful boat you've got there.... are we likely to see the real thing on the water soon?

Been playing around with the hydrostatics and balance some more, with the result being Version 14 (below). The shallow-deadrise planing surface is a bit wider at the transom now, and is carried farther forward, for an effective bottom area about 10% higher than before (thus correspondingly lower bottom loading). There's also a bit more width in the bow, but the overall beam has been trimmed 2% (with gunwale rails, she was just a tad too wide to trailer before!).
Now that I have some rough layout plans (not going to post them just yet, they're still pretty rough and blocky), a more detailed weight balance is putting the CG at around 40-45% of LWL forward from the transom (ie, 55-60% of LWL aft of the entry). CB in level trim is about 10-15 cm aft of this, so she'd trim very slightly down by the bow at rest. Still looking at 3500 kg loaded, around 2500 kg empty.
Spray rails should follow the turn of the bilge, eh? I'll try to work this into the next one. Makes a lot of sense now that you point it out, Will.
Is it considered somehow improper to bring the aft end of the spray rail right to the transom below the waterline, thus making it transition from a spray rail up front to a bit of extra planing surface at the transom?
Somehow I'm convinced that a jet doesn't have to be inefficient at low speed, they just tend to be that way when you optimize them based on top end alone (as virtually all common installations are). I suspect I'll have to coax more data out of the jet makers to see if this is actually the case though. My understanding of the reason for the crappy low-speed performance of most jets is that when you optimize for a 30-50 knot top end, you end up with an impeller that can't absorb anywhere near as much power as the engine is producing at 10-20 knots, thus the cavitation, poor thrust and inefficiency. Sizing the drive and impeller for a 15-25 knot cruise, not worrying about higher top speeds, should be possible.

[RANCHI OTTO]

My opinion is that with 3500 kg, the speed of 25 knots will be reached with 250 hp abt. not less.

The waterjet intake openings at bottom doesn't reduce the lift in this area because the water in the duct produces a vertical lift and for this reason normally no deduction on lift is foreseen.

[Willallison]

Unlikely to see this one one the water any time soon Matt. It's one that I did for one of my Westlawn lessons. I did do it originally in the hope that I'd build it, but I'd need to make a few changes to the hull shape 1st. The shape of the 'chines' is similar to yours in that they are radiussed all the way to the transom. I've refined the shape somewhat now so that they become hard from about station 6 to the back. This should lower trim and power requirements somewhat.
I'm also considering a more contemporary look. Currently though, as I said before, I'm working on a trailerable boat. 9m LOA with a target displacement of 2800kg. This one I've used all developable surfaces for, to simplify construction, so unfortunately I'll have to put up with the chine slap if it ever gets built (or rather, the complaints from the 1st mate that she can't sleep! ). It's still early days for that one, so I'm not nailed down on the chine setup yet. I'll post some images as soon as I get a chance...

Which brings me to the question of your boats weight - how do you come up with 3500kg - it seems pretty heavy. Apart from the additional power required, you have to consider that by the time you sit it on a trailer, the total mass will be approaching 4500kg, which will require something bigger than your average 4wd to tow....

[marshmat]

Which brings me to the question of your boats weight - how do you come up with 3500kg - it seems pretty heavy.

The standard weights-and-moments spreadsheet method, summing the estimated weight of every individual component. (Using some aggregate guesses here, too, since the structure's not designed yet.) The 3500 kg is at maximum load, it's more like 2500 kg dry, and I think many of my weight estimates are rather on the conservative side (ie, she could very well end up somewhat lighter when all is said and done, but probably no heavier).

[marshmat]

For those of you who have yet to realize that I'm probably a hopeless cause when it comes to things marine.....
Hull #16!
In the interests of getting a bit more lift in the stern to keep her level in the lower speed ranges, the chines have been sharpened up aft. Still a soft chine hull for the most part but after playing with control point weights for a while I've elected to take the advice of those more knowledgeable than I and flatten out the chines at the stern. It's more or less identical to the last version but with varying point weightings along the chine.
Spray rail and gunwales are in place now, too.... (this is about the 20th, maybe 25th attempt at getting a spray rail that would both work and look OK on this shape). Opinions welcome, as always
(I should probably point out, BTW, that my naval architecture training is absolutely nil.... what I know I got from books, from studying the boats I see and use, and from paying attention to those who really do know what they're doing; I'm just a senior engphys undergrad student with a love of boats and a tendency to read far too much )

[longliner45]

hey marshmatt,,I see your love of boats,,,,and you put much effort into it.do you want 10 meter trailorboat?because of its size? maybe could put more technology into the trailor itself?,,,,,that is what im doing ,,32 ft,sailboat standart keel, 4ft11inchs ,10.500 lbs ,,,great lakes(3hrs) or chesepeke bay(12hrs) or gulf of mexico(14hrs),from enon ohio....,longliner

[marshmat]

Thanks longliner Sounds like an interesting and very practical boat you've got going there.

As far as trailers go, I like them solid and reliable.... no monkey business, just good quality metal, good axles and brakes, a wiring system that doesn't short out.... (if such a thing exists?)

Why a trailer boat? With my current (trailerable) runabout, I can get to all sorts of neat inland lakes that marina queens can't reach. I can get to neat, remote cruising grounds, and be back in time for class/work. I want to keep that ability.
Why this size? It's light enough to tow with a normal pickup (or in my case probably a pickup with a homebuilt diesel-electric drive) on a normal licence, but large enough to take a decent size group of friends out for a day, or to comfortably live aboard for a week or two.

[Willallison]

Now we're getting there!
I see just a hint on concavity in the bottom fwd sections. Ideaaly you would want convex, or failing that straight sections throughout. It lessens the tendency to bow steer and softens the ride. Post a shot of the waterlines - again, they should be convex as they approach the bow....

"Chine" rail looks good.....

Wish I'd had a chance to grab an image of mine - you'll be surprised at how similar they look!

[Guillermo]

Matt,
In my opinion you'll find 8º deadrise at the transom is somewhat low for a jet installation, making for some tendency to broach with a following sea, as I have found with one of our clients' boat. You'll probably need to add a couple of fins, very much in the way of Tad's. See http://www.boatdesign.net/forums/sho...&postcount=179
Other possibility is to increase deadrise at transom.
I tend to agree with Otto about the power needed. I also think it's going to rather be in the region of 250 HP.
On the lifting thing, I would appreciate very much Otto referring me to some good book on waterjets where to study what he states. I understand the change in direction of the water flow through the jet duct provides some upwards lift, but I'm surprised about this being of the same amount as the loss of lift due to the intake.
Cheers.

Originally Posted by Guillermo

Matt,
In my opinion you'll find 8º deadrise at the transom is somewhat low for a jet installation, making for some tendency to broach with a following sea, as I have found with one of our clients' boat. You'll probably need to add a couple of fins, very much in the way of Tad's. See http://www.boatdesign.net/forums/sho...&postcount=179
Other possibility is to increase deadrise at transom.
I tend to agree with Otto about the power needed. I also think it's going to rather be in the region of 250 HP.
On the lifting thing, I would appreciate very much Otto referring me to some good book on waterjets where to study what he states. I understand the change in direction of the water flow through the jet duct provides some upwards lift, but I'm surprised about this being of the same amount as the loss of lift due to the intake.
Cheers.

i agree on the jets I have done, the owners put on autopilots, even with twins, the bloody things wander all over the place, you have a constant deadrise which is correct

[Willallison]

Hmm - Hinckley's much acclaimed picnic boat has a transom deadrise of about 15 degrees, so the boys might be onto something there...
The problem is that as you are aiming for low speed efficiency, low trim angle and no discernable hump, the lower deadrise would generally be considered better. But with nothing in the water to give directional stability.....

see here for more on the Hinckley: "Picnic Boat" Hull shapes

Originally Posted by Willallison

Hmm - Hinckley's much acclaimed picnic boat has a transom deadrise of about 15 degrees, so the boys might be onto something there...
The problem is that as you are aiming for low speed efficiency, low trim angle and no discernable hump, the lower deadrise would generally be considered better. But with nothing in the water to give directional stability.....

see here for more on the Hinckley: "Picnic Boat" Hull shapes

I built one with quite a long box keel, twins, and it still wandered, they are a NIGHTMARE, unless being used on rivers, whcih is what they were designed for

I posted abt the trailer which you ignored I posted abt the jets which you ignored , well this is me a long time ago before you even heard word jet
Lots of theory in this place, bugger all hands on

[marshmat]

The Hinckley's one of the boats I took inspiration from in this shape.... it's a beautiful boat, unfortunately it's also just a tad on the pricey side for my tastes!
The lack of any lateral area on the jet drive to keep the hull tracking straight could be a concern. I think I'd rather use fins to correct this, though, rather than a sharper deadrise which would compromise the low-speed lift.

Regarding the bow sections on this boat- I'll post more images later today when I'm not at work. I've been trying to keep a slight convexity overall but I may have missed something.

lazeyjack- don't worry, I'm not ignoring you at all, I very much appreciate what you've said so far. IMHO, experience beats theory any day, and you seem to have plenty of the former.

[Willallison]

Finally got a chance to grab a couple of images of one of the hulls I've been working on. The images themselves aren't much chop, but they give you the general idea.

This is from a 12 x 3.8m vessel - so a bit bigger than the one we're talking about here, but the same general principal applies. You can see that the hard portion of the chine extends further fwd - probably a bit too far at the moment - and there's considerable convexity in the fwd section - again, possible a bit too much. It's still early days as far as the hull shape goes really.
As I usually do, I've put a knuckle part way up the topsides. This will get rid of any persistent spray that the chine spray rail (not shown) doesn't separate from the hull. It also helps to make the boat look a little sleeker I think....
I anticipate a single sterndrive as propulsion for this one - 350hp. Top speed should be about 25 knots