Playing around with a 10 m trailer cruiser

Of course it should have a jacuzzi! - as long as you cruise in warm waters, just redirect the jet so it fills a watertight cockpit, add a venturi type system so it aerates the water being fed to the jet - and hey presto, instant bubble bath

No I agree, the overall height is still not that high, but the ability to poke your head out is a real bonus - particularly if you're finding your way into an anchorage at night. The retractable hardtop idea is cool - though the Elling version sounds a bit on the expensive side and doesn't really go with your "if it aint there I don't have to fix it" mantra. It might be difficult to achieve whilst maintaining the trad looks too. Why not just go for simple canvas lid? Light, simple, inexpensive...

 

Probably what Willallison is speaking of http://www.nimbus.se/modelPageCoupe.aspx?pageid=437

And this one http://www.rangertugs.com/r/r-25.cfm has a similar layout to marshmat. But as they have the same weigth and beam limit, they have choosen to shorten the boat and have it deeper. So they could hide the engine under the cockpit sole. They have also used small yanmar engine, ones with the lower heigh (under 65 cm for the 125 hp). They also made bossings in the hull for the engine. http://www.c-brats.com/modules.php?...ame=gallery&file=index&include=view_photo.php

 

Matt,
have you considered the 200 HP JetPac engine? If so, what's the reason not to adopt it?
http://www.swordmarine.com/Products/200hpdsl.htm

 

Yes - Nimbus make some of my favourite production boats, though as always, there's some things I'd do a little differently! Even boats that enjoy the best of visibility from their internal helms can't match the vision that's available from an appropriate outside steering station. The ability to poke your head out the roof provides both.

On the surface, the Ranger tug is a quite different beast to the one that Mat is proposing - though at 2600kg it's actually lighter, which I found surprising. I doubt that with the standard jet installation you would have sufficient room to install the engine entirely under the cockpit sole - at least not without shortening the cabin significantly.
You would then step down into the cabin, so have to consider how to deal with that in terms of cockpit overhang.

 

Nimbus - Gorgeous boats, I like them A lot of similarities to what I'm thinking of, but they are wider, substantially heavier, and far more luxurious than this project.
Ranger - A clever and efficient little thing with some features I'm likely to copy. Overall it's a similar concept done in a very different style. It's three-quarters of the length of mine (7.5 m versus 10 m); I wouldn't call it super-light for its size but it's far from being heavy either. Theirs is a semidisplacement hull, and it has twice the draught of my concept (66 cm versus 30-35 cm). It looks like they put the engine completely under the sole- making the cockpit fairly high. I'm leaning more towards having the engine box protrude into the cockpit, but as a seating/sunpad area.
JetPac - I can see it being a really good option for something a bit faster than this. But taking 300 kg out of the hull at 1.5 m forward, then adding that weight back on at 0.5 m behind the transom, seems to make it hard to keep the LCG of this boat far enough forward to plane at low speeds. I'd probably have it high on the list for a slightly smaller, faster boat though.
I don't think an opening in the hardtop has to be expensive or complex. Electro-pneumatic actuation isn't on the must-have list; I'd be quite content with a big panel that just unclips and slides aft by hand on minivan-style roller rails. Canvas would work, of course, but where's the engineering fun in snapping on a piece of cloth?
The Hamilton HJ274 is the drive of choice for design purposes at present; I haven't ruled out similar jets from other makers of course. A Duoprop or Bravo-3 is of course also possible, although I'd rather have the jet.

 

In terms of large overhead hatches, many of the manufacturers make off-the-shelf items now. Lewmar, Taylor and others spring to mind

 

That in itself is the engineering challenge - to come up with a better system. Just coz it's canvas, doesn't mean it has to be awkward and crappy.
Beneteau had quite a good system for their Ombrine range. The canopy folded down forwards, into a recess in front of the screen. When it was lifted it slotted into something on the screen top and then tensioned aft.

 

From what I understood, Mat project is 2500 kg on the trailer, 3500 kg in running trim, full tanks.
For the R-25, the only figure quoted is 2600 kg. But given what they put inside, I assumed it was empty/trailer weigth.

 

The nimbus 280 coupe I have put the link is 3000 kg, beam 2.85 m. OK wider and heavier, but not that much. But it is no longer in production.

 

I know that critics is easy, and art difficult. And I also know I failed my own attempt.

What I would say on that design is that berths are narrow, if they are double. I would expect the converted dinette to expand up to the hull planking to provide adequate width when used as berth. I would use kind of removable back seat along cabin side. With back seat, seat width up to cabin side. Without, berth width up to hull side.
IMHO, U shaped dinette are only useful if you need the space under the central bench (the bottom of the U). Otherwise, it makes a very small table and do not provide much seating space (no places for foots/legs). Face to face dinnette provide more space.
The galley is on the narrow side. I guess the galley counter will be around the heigh of the catwalks. So the usable depth of the galley counter will be limited by cabin side. It provide a very narrow space for a stove/burners. There is a safety issue if the burners are too near the cabin side.

About the corner in head, where you suggested to put a basin. If you put a basin there, you would like to be in front of it to use it. That mandate the trone to be in the front startboard corner of the head.
I would put a locker of shelfes instead. I would put the trone aft, between engine box and side, and basin in the corner side / front bulkhead.

BTW, http://www.roughwater.com/content/images/29/brochure/29_RW_Family_Cruise.jpg another idea for a boat (empty 2800 kg, loaded 3600kg). They removed the inside helm, but put a two level roof to have a enclosed helm station above the engine.

 

I'd put the basin over the box, and the throne next to it, in the aft stbd corner.

Yes - I agree that the cruising weight of Mat's boat and the R-25 are going to be quite similar, though the boats are 5ft different in length. I'm still not convinced though that Mat's boat needs to be that heavy, given the KISS principle that he's adopting.

BTW - Mat - when are you going to give this boat a name, so we can stop calling it "Mat's Boat"?

 

That's the current estimate, yes- 2.5 t dry, 3.5 t in full-load condition. Since I don't have the actual scantlings yet the structural weight (1500 kg) is still plus or minus about 30% but based on 15-20mm thick strip plank hull with appropriate frame/stringer structure that's what I estimate at present.

I agree with you; of course, my approach to design is to assume the worst, and then be thrilled if it comes out better- rather than assume an ideal case from the start and have to make it heavier later. Nothing wrong with being a bit conservative on the initial estimates.
I would use kind of removable back seat along cabin side. With back seat, seat width up to cabin side. Without, berth width up to hull side.

Interesting thoughts. I've experienced the U-shaped dinette in camping trailers of similar interior dimensions and I don't mind it that much, but your idea of nesting the 'berth mode' partway under the catwalks would be a much more efficient use of space than my earlier sketches.
I'm not sure I'd even bother putting a stove in the galley. I've done long stints without using one before and don't really miss it, so long as I can grill a steak or burger somewhere else now and then. For a day or weekend trip a fridge, sink and microwave would seem to be sufficient; any longer trips would definitely include some shore time.

All in good time, my friend, all in good time...

 

One of the best things about strip planking is that you don't need much in the way of framing. Obviously the engine has to be bolted to something so there's a couple of stringers to consider, but other than that, bulkheads and bonded in place interior fixtures should be sufficient.
BTW - what 'rules' are you using to calculate your scantlings?

 

That decision has not yet been made; I haven't designed the final structure yet (the estimates I have are based on what I've seen in similar boats and on some simple first-principles engineering calcs, not on actual scantling rules). I am looking for an appropriate rule/guide for strip-plank planing hull structures (suggestions welcome).

 

Well, without doubt, the simplest would be Dave Gerr's Elements of Boat Strength. He also includes a development of Lindsay Lords rules, which are considered to be some of the lightest.
In genreal Dave's are considered pretty conservative, but I guess at the end of the day you'll wind up with a structure that you can be confident in. A recent edition of Proboat had an articles comparing his rules for foam cored hulls to the ABS rules and there was stuff all between them.

Apart from 1st principles, I'm not aware of any others.... but like you, I'm always open to suggestions