Riva aquarama replica with, aluminium hull and jet pump

[FinnJet]

Hi everyone,

I am Mechanical Engineer from Finland and i have been working with welded aluminium boats many many years. I have also owned several jet boats and welded one aluboat by myself. I have been planning to build an aluminium boat for a long time. Now i have Riva Aquarama replica plans and i am planning to build it out of aluminum. The hull construction is not a bromlem, but i have a few questions.

All my jet boats have been build with monohedron hull and i know it is ideal with jet pumps.
These plans are similar to the original Riva and the original is made with Warped hull. In the end deadrise is about 11deg and in the middle of the boat it is half more, about 22 deg. Im not a boat designer i just know how to build aluboats. I am wondering if this hull type is goin to work with jet pump. Any opinions?

BR
JR

 

[TANSL]

Welcome to the forum.
Everything, or almost everything, is possible, but it's another matter whether the result is as efficient as desired. I believe, however, that transforming the bottom (new design) to adapt it to the jet doesn't pose any major difficulties. The visible part of the boat, above the waterline, would be exactly the same as the current one, and the submerged part would change as needed. The bottom structure, frames, and longitudinal girders would also have to be changed, and perhaps something else. The new weight distribution would have to be reconsidered and corrected as necessary. Nothing that can't be done without significantly altering the original, visible design of the boat.

 

[seasquirt]

Hi FinJet, I don't see any major problems except for the ugly protrusion you will have at the stern, reverse bucket etc. disrupting otherwise beautiful boat lines. With a trim adjustable jet, and a not too heavy engine, it should be easy to make it power along in nice trim.
Panel vibration transferred from the engine, drive shaft, and jet would be my greatest concern in an a smallish aluminium boat.

 

[montero]

Can you tell me more about this boat? Is it 1:1 scale? What engine(s) and jet unit(s) will be used ? Maybe it would be nice to finish it in a polished riveted aluminum style. Just for the look. Freshwater ?

 

[baeckmo]

With the high thrust line of the jet there is a risk you will experience directional instability and broaching due to low trim angle. You may find images of this hull running with the bow wetted. There you can see that the bow spray is following the bottom all the way up to the chine, where it separates without downwards deflection.

To improve the trim issues, the best way is to convert this spray to a lift by adding two spray-rails up front and possibly also add a horizontal strip to the chine forwards. Note that there should not be any disturbance within the bottom area leading into the jet inlet, the lower rail only about 1/3 of WL length, the outer might go all the way to transom for added directional stability (at the slight cost of increased drag, though).

 

[montero]

Trim of the jet is relatively easy to control , in some range.
The shallow depth of the hull at the stern , does not allow for mounting the pump/engine at the right angle?

 

[montero]

That bow spray isn't it mainly due to the strong weight of the narrow stern?

 

[wet feet]

Some interesting points have been raised but I don't see anything directly related to the issue of the hull not being a monohedron form.My thought would be that the original hull shape seems to have worked well enough,but the weight distribution of the proposed boat would need to match that of the original and the thrust line would necessarily have to be quite close.Otherwise,to alter the immersed shape and then style the topsides to match the original,would effectively be a different design and would necessitate a complete new set of calculations to be applied.Am I wrong?

 

[montero]

Some interesting points have been raised but I don't see anything directly related to the issue of the hull not being a monohedron form.My thought would be that the original hull shape seems to have worked well enough,but the weight distribution of the proposed boat would need to match that of the original and the thrust line would necessarily have to be quite close.Otherwise,to alter the immersed shape and then style the topsides to match the original,would effectively be a different design and would necessitate a complete new set of calculations to be applied.Am I wrong?

What I would do first is move the engine as far forward as possible. Only O.P. knows what engine and jet unit to use.
The Rive was built somewhat according to the old style of hydrodynamics.

 

[FinnJet]

Thank you all for the good discussion. Under no circumstances am I going to modify the hull of the boat. I am going to keep it 100% original. If this really goes wrong, i will then re-install a sterndrive. But I have done some research. I am planning to use Hamilton jet HJ212 wit 350hp v8 engine. I know it is a bit small unit, but my friend had it once in a same sized aluboat and it worked perfectly. I am going to use trimmable nozzle so i can trim the spray angle / boat runnin angle. I went to Hamilton Jet's website and there was few good points about suitable hull types. If i understood it correctly I think that there is no broblem to use jet pump with this hull.

 

[seasquirt]

You can trim the hull by the length of the drive shaft used, placing the engine in the optimum position, then using the trim nozzle for final adjustment according to the no. of people on board, fuel load, and size of picnic basket, etc. With a fuel injection system and iridium plugs it might almost be fuel efficient and smooth running. Could even turbo it mildly. I love the look of those classics, and the sound of a sweet running V8.

 

[BlueBell]

FinnJet,

Was the design plan you are building from, ever built?
If so, how did it perform?

By the way, it won't be a replica but rather a recreation.
I realize English is not your first language, I'm not criticizing.
Just clarifying.

Nice project. What's your completion date and budget?

 

[seasquirt]

FinJet, I had a devious idea you may like, for after your build, before any corrosion occurs: epoxy paint seal it, (or whatever system), then get wood grained wraps applied with intricate patterning and features/highlights, better than the best wood boat, so that at a distance any wood boat officianado would be thoroughly impressed with the looks as you speed past. A good artist of wraps could do a very convincing job. Only very close up would the truth be known. Giving extra metal protection, having matched patches for scrapes, and every few years you could do a make-over, and look brand new again. No wood working required, unless you wanted some real wood for some upholstery trimming.