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Old 03 June 2004, 17:20   #1
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Country: Germany
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AVON Supersport 4.65 with Jet?!

Hello,

me again. I`m thinking about something special and would like to hear the opinions of all the experts in here:
I went out with my RIB today to test the effect of some Doel-Fins that I`ve mounted on my 75hp Honda because the boat started to become nervous atop 33kn! What happened today? In some ways exactly what I`ve predicted: The boat lost 3kn topspeed, normal due to enhanced drag. The boat now started getting REALLY nervous (dangerous?) from 30kn on, it did what we call "sawing on the outer rails"- so this is a complete desaster? No, because while at displacement- speeds the boat performed much better when I trimmed the motor down, it really lifted up his (too) fat a..! It also had a much smoother pass from displacement to planing, not bad! Since I`m more interested in cruising I`ll leave the fins on- hate going over 30kn on the water in a RIB anyway.... don`t get me wrong, while I was in the german Navy I was on a fast attack boat which was capable of going almost 50kn!!!!
So everything was fine until I looked into german Ebay, look for yourself:

http://cgi.ebay.de/ws/eBayISAPI.dll?...480887510&rd=1

Bought that one right on the spot, will pick it up on the weekend. The jet is powered by a little FIAT engine with 1000ccm and about 40- 45 hp. It`s pretty easy to squeeze another 10- 15hp out of them, just a pair of Weber carbs and a sharper crank will do.... so what about putting this into my Supersport?! Has anyone in here already done something similar? Here are my thoughts:

1. I`m losing weight, the jet-unit should be about 40kg lighter than my Honda!
2. I`ll have the center of gravity moved forward= better trim since the engine doesn`t hang on the outside of the transom but is located on the inside!
3. I`ll keep 4-stroke technology with very cheap parts- you`ll find thousands of these engines in cars
4. I`ll have something special

I`d really like to know what you guys think about this, shouldn`t be so complicated to swap that unit into my hull since I have the templates from the old hull!

greetings,
Wolfie
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Old 03 June 2004, 17:34   #2
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Wolfie, might be easier to cut the hull of the jet boat down and stick some tubes on that.

Make an interesting project if you do it.

Pete
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Old 04 June 2004, 02:56   #3
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Hi Wolfie

Can you explain why your actually going to do this ?

From what i understand you want to take the jet drive and engine out of this other boat and instal the whole assembly into a 4.65 metre Avon ! WHY ?

You've already stated that the Avon starts chine walking badly above 33kn, so i'd suggest this was about the maximum engine HP/weight ration for the hull !
When the Avon was designed and built it was intented to carry an outboard, you may effect the dynamics of the boat by bringing the engine inboard in this way, plus for the ammount of work involved is it going to be really worth the hassle.

Rat

PS if you throw that engine i'll gladly take it off your hands
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Old 04 June 2004, 03:39   #4
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Hey Rat,

yeah, you`re right- I want to step down the hp and also the weight, that`s why I`m thinking about this procedure! The 75hp HONDA is to heavy for the boat although it`s performing very well at lower speeds. I think the engine you ride on it is the perfect match and since the V of the hull is rather moderate it doesn`t eat up that much hp`s! I`ll pick up the jet boat on the weekend and after that I`ll have a close look on this experiment, will post some pix to let you see it!

greetings,
Wolfie
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Old 04 June 2004, 03:50   #5
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Quote:
Originally Posted by ribfreak
Hello,


I went out with my RIB today to test the effect of some Doel-Fins that I`ve mounted on my 75hp Honda because the boat started to become nervous atop 33kn!
1. I`m losing weight, the jet-unit should be about 40kg lighter than my Honda!
2. I`ll have the center of gravity moved forward= better trim since the engine doesn`t hang on the outside of the transom but is located on the inside!
3. I`ll keep 4-stroke technology with very cheap parts- you`ll find thousands of these engines in cars
4. I`ll have something special

I`d really like to know what you guys think about this, shouldn`t be so complicated to swap that unit into my hull since I have the templates from the old hull!

greetings,
Wolfie
So as I understand this you want to take a perfectly good working Rib that as you say goes fast enough & put an 'unknown / untested engine combo into it with the view to making it what?

1. I`m losing weight, the jet-unit should be about 40kg lighter than my Honda!
Solution - carry 1 less person - cheaper to do this..

2. I`ll have the center of gravity moved forward= better trim since the engine doesn`t hang on the outside of the transom but is located on the inside
Solution - Surely the boat was designed with this transom to achieve this?

3. I`ll keep 4-stroke technology with very cheap parts- you`ll find thousands of these engines in cars
Solution - You may need them as 'car engines' don't mean they work well as boat engines.

4. I`ll have something special
Solution - Unsellable

Surely better to either sell this complete boat & use the funds to buy another hull only if you are that keen on this idea.

Not really keen on this idea at all, but good luck if you do decide to try it.

Pete
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Old 04 June 2004, 05:29   #6
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Wolfie... i have to agree with pete on this one (although he's nuts )

Someone once spent a lot of time developing and testing this type of hull to its maximum and with different engine/weight combinations and now you come along and deside they were wrong and your right, why chop up a perfectly good transom to add an inboard engine.

If you still decide to go down this route i think you should first talk to Avon, i wanted to chop a small hole in the fore deack to fit a hatch into for stowage, Avon advised against cutting into the deck as there is A, insufficent space between deck and hull and B, the deck is an integral part of the structure of the boat. I dont know very much about inboard engines on RIbs but from what i've seen they seem to be sunk down into the hull, i assume this means the centre of gravity is lower and the weight of the engine is as far back as possible.
Pete's right, if your not happy with the boat or its performance then sell it as it is and find something you are happy with, cutting it about will only make it unsellable to someone else, if it goes wrong your'll end up with a complete Monster. If your finding your current configuration to fast and unstable then simply throttle back a bit, who says you have to go bat out of hell all the time (dont answer that Pete, we know your nuts ).

End of the day though if your still commited i'll still come and get that Honda 75 off you
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Old 04 June 2004, 08:19   #7
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I reckon it sounds like a fun project if you've got the time to spare and can afford the risk of ending up with two unusable boats!

You could end up with a cool little jet RIB, but this sort of project rarely turns out to be as simple as it looks!

Let us know how you get on though

John
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Old 04 June 2004, 09:53   #8
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New idea

I have a better idea..... how about making extended skirts out from the tubes and fitting in a big fan motor, then you could have a hover RIB, you'd solve the obvious problems of launch and recovery and be able to go practically anywhere you wanted

One day some bright spark will invent retractable wheels for a RIB powered by a small electric motor, then as you approach the slip way a quick flick of a switch and the wheels lower down and the electric motor engages and you simply drive the boat out you could then either drive it straight onto a pupose made trailer or if you live close drive it home like a car
You'd never have to replace trailer wheel bearings again
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Old 04 June 2004, 09:57   #9
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And while my creative thought process is running wild, i'd always thought it would be a good idea to have a trailer extension on your trailer, this would slid out from under the trailer and lock onto the existing trailer giving you double the lenght to roll the boat down into the water, means never getting the bearings near the water and you'd not need to get wet launching on shallow slips or beaches.
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Old 04 June 2004, 11:04   #10
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Quote:
Originally Posted by Bilge Rat
And while my creative thought process is running wild, i'd always thought it would be a good idea to have a trailer extension on your trailer, this would slid out from under the trailer and lock onto the existing trailer giving you double the lenght to roll the boat down into the water, means never getting the bearings near the water and you'd not need to get wet launching on shallow slips or beaches.
Now that is a good idea & wouldn't be to dificult to do surely! 2 or 3 extendable rails with rolers on, why hasn't someone thought of it before.
You could make a fortune out of this Bilge Rat, get down the Paintentís office quick.
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