Aluminium jetboat - A couple of questions.

cheers
Lurvio

 

that diamond plate decking looks interesting

 

Hello

I had a long break in boatbuilding as the welding wire took five and a half weeks to arrive. Now I've been working on the boat every now and then after work and the welding is getting along nicely.







cheers
Lurvio

 

Hello

Thought it is time to update a couple of images here too, it's been 518 days since last time after all.





Made the trailer too.






Test float, July 8, 2013


The heat exchanger.




Trailer painted and ready for registration.


Yesterdays score



cheers
Lurvio

 

Thanks for the update. Make sure the copper is isolated from the aluminum. When do you anticipate the first trial run?

 

DCockey
The lakes are just about to freeze over, so I have a few months. Without a major disaster, test drive in may or june.

 

Sweet looking boat
The biggest concern area of concern that I would have is the engine compartment
A marine engine has the distributor, and alternator sealed and spark arrested as well as putting a flame arrestor over the air intake so that if you get a combustible fuel mixture in the engine compartment, it has less chance of being ignited.

Hard to tell where the air intake is for the engine, perhaps down the inside of the engine compartment. I would find a way to get this intake from outside the passenger area so that if you have an explosion, the flame front will not come into this area.

You should have some engine compartment exhaust vents to keep vapors down in any case, and I believe that ABYC/US coast guard guidelines, which do not apply to Finland but are a good working set of rules, required the compartment vents not to be vented into the transom area.

Alternatively, I would mount two blowers to exhaust the engine compartment, that run all the time the engine is running to lower this possibility. Two, only because if one should fail you have a second unit as back up.

They are a very cheap insurance

I would have changed the area ahead of the intake a bit so you did not have the front edge of the intake so close to the transition between the hull deadrise and the flat area.
Normally, jet pump manufacturers want a flat area 2 to three times the length of the intake ahead of the intake. Does not mean that your config will not work but rather it might not be as good as it could be.

How are you driving the pump, vdrive or belt drive?

Want to see some youtube video when you get it running.
Good luck

 

Thanks for the comment Barry

Going to have two fans in the engine compartment, one of them ducted from under the engine. Engine air intake is at the left rear corner.

Next boat might have a different delta, lots of options there.

Pump is driven directly from the flywheel via a coupling (a hardy disc probably). Have to figure out the coupling once the pump arrives.

Got a GoPro-camera on the christmas wish list.


cheers
Lurvio

 

The coupling should be a flexible coupling for a couple of reasons
First it is hard to get the flywheel and the pump intake concentric as well as parallel
Even a few thousandths out of perfect alignment will load the pump bearings unevenly.
The other item will be the odd bottom grounding that you might have.
I am not sure about the ductility of your pump but we had a situation many years ago when a customer hit a rock and cause a deflection of the hull and the pump intake housing actually flew apart, sunk the boat, bad experience.
A flexible coupling will lower some risks here

 

Another item
I did not see lift strakes on your hull
We used to install 3 inch by 5 angle lift strakes, one pair about 1/2 way in as well as turn down the chine by the same amount to the bottom of our boats

Certainly we got some lift from these items but the biggest thing that we saw was an
increase in steering. The strakes tended to cut the water and prevent the hull from sliding in a turn. If you have not ran a jet before, they turn best under power and will not turn if you back the power off. The strakes would hold quite a bit of the slip and if you turned and applied power the effect was dramatic

Do not form a lift strake as the outside edge will be curved and this will actually pull the hull down due to the Coanda Effect
If 3 inch by 5 inch is to aggressive, you could cut the 3 inches down to 2 inches. On a 12 degree hull we would use a 2 x 5 and on an 18 degree the 3 x 5 appeared to work best
Of course the 5 inch is the almost flat portion and the 3 inch or 2 inch is the almost vertical leg.
This piece must be welded on the hull bull length, as compared to being stitched so you do not lose the lift ability by bleeding off pressure. Some manufacturers stitch weld this on and leave the back of the strake open. This is almost counter productive for lift, though the steering help would still be there

 

A hardy disc is a rubber donut with six bolt holes. On automotive application they have a center pin that centers them and carry the weight of the shaft to the transmission bearings. I plan to not use such centering so I should gain a reasonable 'limp home' ability from the coupling. I can adjust the alignment pretty darn close with a simple alignment shaft so I am not that concerned about that.

Jet intake is made out of ali plate.

I thought I had a link posted to the build thread on nzjetboating, but seems I was wrong.

Lurvio's jetboat - Nzjetboating.com

Edit: Just saw your second post. I want to test drive the boat before adding strakes. The NZ guys seem to be using strakes for steering only as they are usually a lot narrover than what you are succesting.

cheers
Lurvio

 

Most of the jet boats that we built were in the 18 to 23 foot range and deadrises between 12 and 18 degrees, and we wanted to maximize lift, hence a wide strake.

There is not doubt that you will be able to statically get the shafts concentric but there will be flex of the hull shell as you are pounding down the rapids and perhaps tagging some rocks. Small deflections will cause some concentricity issues between the two shafts. This can cause some significant side loading of bearings.

When I went back to look at your pictures I noticed that near the engine mount, you had a flat plate that went to a center of a floor plate. I could not see it all, ie how you did the welding, but I would watch this area closely for fatigue cracking. The center of the plate will be flexing all the time and this "hard spot" could cause some stress concentrations with resulting cracking due to fatigue.

If you are building another, and you are going to put in this type of member again, you could weld say a 3 inch by 3 inch flat by 1/4 inch material flat against the hull, then weld the member to it.
Alternatively, try to avoid short weldments in the center of a plate which will be flexing.

We also reinforced the keel on the inside to keep the outside keel clear of increased turbulence

Around the pump and under the engine, we would add in 3/8 to 1/2 inch plate, flat to the hull due to engine oil pan clearances.

Then ahead of this we had two options, either continue the two flat 5 inch by 3/8 to 1/2 inch forward to where the keel began to curve upwards, or alternatively run in another stringer starting as close to the engine as we could

Normally if you hit a rock off the center of the keel the boat will heel over, reducing damage.
But if you hit right on the keel, this heeling action cannot happen, so you rely on the keel to take the whole force of the impact.

Cheers

 

Those stub frames are something I didn't plan very well and might not be even needed. On the other hand I need to support the outer side of the stringer somehow around the engine mount. I think I'll make two sided stringers (upside down U-section) for the next ali hull.

I've been thinking about a keel bar and might still add one.


cheers
Lurvio

 

Well done, Lurvio!

 

another jetboat idea...

Lurvio- well done on your design how did it perform?

thought of something similar but at about 4.7 meter aprox. (16 ft)

beam 6 ft
draft .9 ft planed draft aprox- 4 -6 inches

the seat has been moved forward to accommodate 2 people.
planned 350 - 400 hp
Scott jet 8.25 or Hamilton 3 stage or turbo impellor...

tell me your thoughts?