Hi Tim

They are prone to sitting bow down, partly because the bow design doesn't offer much support. As you say Dave's build was Ok though.

The bow will come up fine under power but try and come off the throttle gradually rather than suddenly when on the plane.

Pity to add ballast, especially as Dave's mantra was to keep it light but 150g isn't that much. Is there any scope for moving the battery back?

I can't wait to try my 1:12 version in the bath and see how it sits – needs the prop shaft and rudder installing though first.

Chris

Edited By Chris Fellows on 19/07/2021 11:54:59

A well known personage came to the pond ages ago with (possibly) the same kit and I commented on the bow down look. As per comments, it rose up and looked fine once moving and went very well.

Ashley

Yes, Dave's prototype sat level at rest but others have reported the bow down attitude of their kits. I suspect it is due to the quite subtle bow section where there is a tendency to carve off too much material from the balsa bow blocks to achieve the Fairey double curve.

I tried putting a bit of ballast in the stern of mine and although it didn't affect the performance noticeably it did make the bow come up rather too much at speed so I removed it.

Colin

My boat uses Dave's specifications to the letter.

Colin

Nothing to be done about it really, you just hav to accept it. No problem with the beamier Swordsman.

Colin

Dave and I compared notes at the Mayhem meeting in 2019 and there seemed to be no obvious reason for the discrepancy. He was as puzzled as I was.

Colin

The Perkasa prototype and supporting information was sent to SLEC late last year so hopefully it will still appear as a kit and tribute to Dave.

Colin

Hi All,

I don't think the weight distribution comparison between fullsize and models is a reality as all we have to play with is the position of the motor & battery as opposed to big petrol or diesel engines, It's all a compromise at the end of the day

Regards Ray

Please do not think I am critisising anyone here, I am simply adding comment for discussion!

I think something that is frequently overlooked is the complete difference between static stability and dynamic stability. Every vessel on the water, model or full size, is designed to be at its most stable at its normal operational speed. All other speeds are therefore a compromise. Many real life vessels can be difficult to manoeuvre because their hull form is designed to be its most efficient when moving at normal speeds.

What I am therefore basically suggesting is that the stability at full speed is far more important than the way it sits in the water at slow speeds. I am sure that Dave took this into consideration with the design so if my model looked a little bow down when going slow I would not be concerned. Colin's looks perfectly set up at speed with just the right amount of hull clear of the water.

As already mentioned, I think there is a tendency to remove too much material from the bow blocks and make the section a bit over concave. It is quite easy to overdo the balsa butchery….

Mine looks fine when it starts moving, it only droops when it stops!

Photos of the full size Huntsman show that it sits level at rest.

Colin

Hi all,

Yes the H31 sits very low at the stern because of the engines weight, look at the waterline painted on this one which was in our local yard a couple of years ago.

Regards Ray

It is very hard to imagine a designer of a full size boat being happy with producing a bow down attitude at rest. I am not talking about about extreme or racing boats but boats that people buy to go cruising in. The normal datum for a boat is either the waterline or an arbitrary line parallel to the waterline but below the deepest part of the keel (the latter most common for motor boats). All the bulkheads, interior joinery and the cabin soles will relate to the datum either being at right angles (bulkheads) or parallel (cabin sole). If the weight distribution is wrong and the boat is bow down against it's designed waterline at rest it would be very noticeable inside and unnatural. It is very odd and slightly uncomfortable and would not be tolerated.

Fore and aft trim at rest is therefore important. Likewise it is not good to have a permanent list. Trim errors at the sort of angles we are talking about have very little effect stability. A boat can be significantly out of trim and still be perfectly stable. The extremes however can be quite dangerous and a classic example will probably be familiar to those who sail dinghies or use speedboats. If you climb onto the foredeck and the stern comes out of the water the boat becomes unstable. If you mate then joins you, you are very likely to capsize. This is because you have made the waterplane very narrow at the bow and useful waterplane is now in fresh air.

Richard is right when the subject moves to dynamic stability and that is very often optimised for a certain speed range. A planing hull can be quite tender at rest and considerably more stable when on the plane. Push it too hard though and there is not enough boat in the water to provide the stability and it may get very twitchy, either chine walking or porpoising or BOTH. Round hull semi-displacement boats that may be perfectly happy up to say 20 knots and can, and have been known become dynamically unstable if pushed to far. Very scary!

In my experience trim is a good word to describe how a vessel sits at rest and attitude best describes the position the boat adopts when the boat is moving. I think it is quite useful to make the distinction.

As far as adding weights is concerned if there if there is nothing left to move, it is almost always better to move a lighter weight as far away as possible from the longitudinal centre of buoyancy. The shorter the distance the more weight you have to add to shift the longitudinal centre of gravity.

I have the Precedent kit to be fitted with an Enya 35 marine glow engine. The instructions warn about the tendency for the complete boat to be bow down at rest (out of trim). For this reason I have worked to get the engine about 40mm back from where it was shown on the plan and everything else will have the possibility of going aft as far as possible. Let's see if that works.

Tim R