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24" Scratch Built Mono Hull Nitro

5K views 12 replies 5 participants last post by  mouse 
#1 ·
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Scratch build, I hope that is the correct term.

I built this boat about 20 years a go from plans I bought from Float a Boat in Melbourne.
This has only been in the water a couple of times and it would porous a lot once I got the speed a little over idle. I did try putting some weight in the front thinking it was to light in the front, but it went to not being able to get it out of the water on to the plan. I did this about 5 or 6 years ago and have not touch it since.

After reading in a thread on here about how to work out the COG (the plans had no COG on them). That inspired my to get the boat out from under the house and have a look see. I worked out the COG for my boat to find that it was 5cm to far forward and on a boat that is only 62cm long I think that is out by a lot. I did some weight testing to find that some how I need to get 750g of weight on the rear end to get the COG correct. Being a small boat there is very little room it the radio compartment to do this so I am now trying to work out how I am going to fit this weight in or on the rear.

Would the heavy front end be the cause of the porpoising?

I am not able to move the engine towards the rear much if any, due to the shaft length.

Thanks from any thoughts.
Col.
 
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#2 ·
Yeh to me it looks way to forward.....to stop the porposing i would fit some trim tabs perhaps. If that dos'nt work then i'd be removing the engine ,stuffing tube and mounting the engine back

Good luck with it

Cheers
 
#3 · (Edited)
Just another thought. Have you researched the drive angle of the prop. This can cause the nose to rise or fall on power. It sounds like the prop shaft drive angle maybe to horizontal to the bottom of the boat. As i understand the front of the boat should lift itself out of the water and the rear should be pushed up a little by the prop causing the boat to balance and ride flat . If the shaft is to flat it will lift the front too much then become unstable and falls again causing the porpoising you talk of. This may also explain why you initially thought it was too light in the front.

Taken from http://www.tropicalboating.com/powerboating/tilt-and-trim.html
This is for outboards but I read "tilted too high" the same as 'too flat a drive angle'
Tilted too high

If the engine is tilted too high, the bow of the boat will ride too high, and the stern will "squat" in the water. Many boats will start porpoising in this condition. The tilt of the engine lifts the bow too high, the bow falls down, it gets lifted too high again, and so on.
Not sure bout your cog tho. But 750gr sounds like a lot. I would start by swapping the electrics and the fuel tank round. Although the COG would alter as the tank empties.
 
#4 ·
Another option is to add spray and/or chine rails.

A typical monohull design (stepped or conventional) will incorporate devices known as "spray rails" which deflect water downwards and away from the boat, both to reduce wetted area (and thus drag) and to generate lift to make the boat plane and to keep it planing when running at speed. The edge of the boat where the sides meet the bottom is known as the "chine".

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#5 ·
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Thanks for the input guys.

The boat already has chine rails on the hull but not right out on the edge of the hull. Do you think it would make any difference fitting ones on the outer edge?

If I under stand the prop/shaft angle write up correctly I think it is saying if the shaft angle is pointing upwards it can cause the porpoising?? My drive shaft is a solid one and is angled down, so would that be trying to push the nose of the boat down??

I have made up some dodgy trim tabs, so I think I will try mounting them on the rear and see if that makes any difference. I will also try to get some weight in the rear just to see how the boat reacts to these two changes.

Due to the shaft being solid and the room around the flywheel area I don't think that moving the engine and shaft backward is going to be an option, so I will try the tabs and weight first.
Thanks
Col.
 
#6 ·
Definitely add trim tabs, you can fab yourself from alloy and make them not just an L shape but give them a lip at top so you can screw down bolts for more angle if needed...

And dont get a lifter prop, mono's run semi submergered....cheers Check the rear of my CF race mono, in for sale section.. Good view of rear... pm for help if needed
 
#7 ·
I am a complete noob to boat but surely the Negative angle of the prop is causing huge wet area and that is not such a good thing?

Shouldn't the angle be closer to neutral?

Also porpoising can be caused by strut to shallow or close to transom.
 
#8 ·
When I get a chance to take the boat out for a run I am going to try two things, the trim tabs and then adding weight to the rear.
First the dodge trim tabs I have made. Going by the hull attitude thread I think I need to mount the tabs towards the center line.
Do you think where I have them in the pic is about where I should mount them or about half way between the center line and side edge? Light Motor vehicle Yellow Red Gas


This is about the amount of lead I need to put in the rear to get the balance point. I will add a little at a time, but I think it may just get to heavy and not get up on the plan. But I will just give it a go and see what happens. Wood Cable Road surface Rectangle Wire
 
#9 ·
For starters its a mono and you dont have turn fins.. Will be great in a straight line.....

To get cog, get a small dia pipe or hose and sit it under hull... you want it between 25 to 28 % from rear as a starting point..

Do that, your tabs are great if u dont wanna turn and unless they are very rigid u would be suprised how much flex they will have under load.. (they look thin)
on the right path and good to see the blood flowing again..
cheers justin
 
#10 ·
The boat is not very quick so turning was not a problem (only has an Enya 15 engine) In saying that I don't know how it will go once I can stop the porpoising and get it up on the plan.

With the amount of weight I have put in so far I got it to balance at around 30 to 32%, so proberly more weight in the rear when I get to give it a run??
Pic of it balancing around 30 ish % Watercraft Boat Boats and boating--Equipment and supplies Wood Naval architecture


The tabs are only tin plate so I may get some aluminum and make something stronger.
 
#12 ·
The boat is not very quick so turning was not a problem (only has an Enya 15 engine) In saying that I don't know how it will go once I can stop the porpoising and get it up on the plan.

With the amount of weight I have put in so far I got it to balance at around 30 to 32%, so proberly more weight in the rear when I get to give it a run??
Pic of it balancing around 30 ish % View attachment 15649

The tabs are only tin plate so I may get some aluminum and make something stronger.
I am no boat guru, but surely you dont want to add weight. Like tiny said, try other things.

Can the motor be moved more towards the rear?
 
#11 · (Edited)
is there any way u can ditch your fuel cell and get a bladder... Will easily get u 5% and reduce weight.... Never want to add weight bud, power to weight is everything... Bladder or rob a hospital for IV bag....
Serious

or mount cell sideways.. Do you have flotation, eg pool noodles up front.....

Ditch it and use sellys space invader expandable foam at rear... Keep elecs dry on roll over and nitro can easily be flushed and oiled if a stuff up occurs.. Electrics cant and will help get COG sorted...
 
#13 ·
Hope to give the boat a run sometime in the coming week, have some days off work.

Justin, I will look at getting a iv bag thingy, but before I can get one I will only be putting a small amount of fuel in the tank. I will also look at if I can turn the tank sideways.

I have polystyrene foam sealed in the nose and under the floor where the tank and radio sit. These sections are sealed and I can not really get to them.

Radio section at the rear is sealed and waterproof by screwing down the clear perspex cover.

l_d, Due to the design and build I can't move the engine backwards. There is not enough clearance under the flywheel and being a solid shaft I can't change any angles. During the build I needed to cut away some of the main center rib to get clearance aroung the flywheel.
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So what I am going to do is try one thing at a time, first will be the trim tabs to try avoiding adding weight. Then depending on what affect the tabs have then I will add the weight to adjust the COG. I know adding weight is not good, (power to weight) but the boat is almost built around the shaft so I would like to avoid having to remove the shaft. I will see what the outcomes are of these first adjustment and then go from there.
 
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