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[Paul R]

Hi.. I have an Aquasoport 80 which planes but starts porpoise as soon as it hits a ripple in the water. I've tried putting extra weight in the bows and alternativeley in the stern of the boat, but this makes little difference. It has a single 700BB turbo running in 14.4v with a 3 blade brass 35mm prop. Has anyone got any ideas as to what else I could try?

Thanks.

[triumphjon]

have you tried different props ,  i normally use 2 bladed fairly course pitch props on faster models , also is your model fitted with trim tabs ?

[omra85]

Hi Paul,
Porpoising is caused by the thrust from the prop being at too sharp an angle.  The power is lifting the nose of the boat out too much, gravity takes over and drops it back, then it starts all over again!
If you have 'surface' drive, try lowering your shaft fractionally. This will push the nose of the boat down.
If you have submerged drive, your shaft angle is too steep.  The only cure is to reset the shaft at a flatter angle.
Cheers
Danny

[triumphjon]

what about trim tabs ? as removeing the shaft may not be posible .  ive seen several plans of late with very steep shaft angles  !

[omra85]

Trim tabs will do the job, but they induce drag. 
The designed drive angle of any boat varies considerably, especially with submerged drives.
Practical considerations such as length of hull, C of G, C of B, engine height, internal equipment positions, prop size, etc, etc, must be taken into account.
Under hull props generate both thrust and lift.  The lift element is the undesirable bit, but must be factored into the drive design.
It follows that, the less lift you are creating, the more the energy can be used for thrust, therefore (as a general rule) the more thrust being generated horizontally backwards, the faster you go.  The hull shape is then what is keeping you on the water.

Danny

[Paul R]

Thanks for your replies guys...

I can't reset the shaft angle without destroying the hull as its buried in half a ton of epoxy!, came ready built There must be something else I can do. What about extra weight in the nose, maybe this would stop it porpoising or shifting the two battery packs that flank the motor another 8 or 9 inches further forward towards the bows. I've tried small adjustments shifting the batts backwards and forwards and I've tried smaller props none of which works. so how about half a pound of lead weight up near the bow, trouble is once there I might not be able to get it back out again. I don't want to sell this boat as its a real looker on the water...

Many thanks

[pompebled]

Hi Paul,

Considering the Vietnamese building quality, getting the stuffing tube out should be easy;
Take out the propshaft, remove the grease as much as possible (or it will drip out) and heat the stuffing tube with a large soldering iron (or a flame) to soften the epoxy.

Once it's hot enough pull the stuffing tube out with a pair of pliers.

Getting the remaining resin out of the hull is another story, that involves a lot of work with a Dremel.

Another option would be to soften the stuffing tube, so you can bend it to be parallel with the keelline (or as far as you can get without having the prop hitting the hull) and use a flexshaft, with a solid endpiece and a bearing under the hull.

Other than that, adding 'brake' tabs to keep the porpoising to a minimum and take the loss in speed might be the only solution to a bad set-up.

Most plans you've seen 'of late' are scaled down from the original, where the steep propangle isn't much of a problem due to the water being on the same scale (1:1), a model doesn't have that luxury.

Scale hulls that need to run quick have a long (very long) propshaft and a very shallow propshaft angle.

Regards, Jan.