Waterline and ballasting for SLEC Perkasa kit

Hi folks, I am building a 30” Perkasa kit as produced by SLEC. I am at the stage of marking out to paint the waterline/lower hull colour. A test float in the bath indicates it is going to need quite a fair bit of ballast to sink the hull to the plan indicated waterline. My question is this is it better to invent my own reduced waterline for the lower hull colour or ballast it down to the plan one and I guess accept some reduced performance due to the weight? Any thoughts would be welcomed. Thanks in advance.

Hi Chris,

This kit was designed by the late Dave Milbourn and was his last project. Normally he would have provided very detailed instructions but I don't think he had the chance to complete the job which was finished by SLEC. I assume that there is nothing in the instructions you have concerning this?

The recommended powerplant with a brushless motor and 3S LiPo battery is pretty poweful but also lightweight. A 'traditional' brushed motor NiMH setup would be a lot heavier so it is reasonable to assume some ballast will be needed.

Perkasa is intended to be a near scale model so if you paint the waterline anywhere other than as indicated it won't look right. It also means that performance should be near scale in appearance too, this is not intended to be a model speedboat and if overpowered it may not perform properly anyway!

Did your bath test include all the internal mechanics and electrics plus all the external fittings and superstructure?

I think your best bet is to provide for removable ballast so that when the boat is complete and working you can adjust it to give the best balance between speed and appearance on the pond.

I built both of Dave's Fairey powerboat designs and reduced the battery to 2S on the Huntsman as it was simply unrealistically fast and unstable on a 3S, even with some ballast aft to improve the trim.

There are plenty of photos online of this type of craft which show what they look like at speed.

Colin

This is a personal view, others might disagree. I would not add any ballast at all, if you can avoid it, maybe just for trimming the model.

I'd do a flotation test, and mark a new waterline, if it doesn't seem ridiculous I'd paint it there. If it does look a bit silly, I'd compromise and lower the waterline as much as looks ok. Once the model is finished and working I doubt you'll ever give it any thought again. Once you see it powering over choppy water and see a graceful high speed turn you won't give a fig for the waterline. Great models and great performers.

Charles.

Chris,

I've just remembered that Dave Abbot reviewed another kit of Perkasa in 2009 and the article included a number of on the water photos of the model in action. You can see it nhere:

**LINK**

The waterline appears to be in a similar place to the SLEC model and the boat at rest seems to be floating down to it.

You can see clearly what the model ought to look like in motion and note Dave Abbott's comments about getting the planing effect just right.

Dave Milbourn wouldn't have put the waterline where he did for no reason, he was a top designer of this type of model. He used Dave Abbott's photos when designing his SLEC kit, I know that because I have just found the email Dave A sent me back in March 2020 to pass on to Dave M!

I would definitely leave the waterline where it is and add whatever ballast you find is necessary. I don't think you will have any practical performance issues with that power set up. I found that my Huntsman handled a lot better with a bit of weight inside as it improved stability, particularly in the turns. It looked a lot better too!

Colin

Thanks for your replies Colin and Charles. Much appreciated. Perhaps I should wait til I have built and added all the externals such as the guns and lockers etc and then do a bath test again and then take a vew. Removable ballast sounds like a good plan if i do add any. Thanks again.

Thanks for the link Colin! Will take a read through. Much appreciated.

Chris,

You might be surprised at how much all those fittings add up to! Dave was always complaining how many there were!

Colin

If you have a look at Page 16 in Albums you will see some pictures of Dave's build and an angled shot of where the waterline is.

Chris

Is this the Album you mean Chris?

https://www.modelboats.co.uk/albums/member_album.asp?a=53845

Colin

Thanks chaps for those links. The pics in the album of Dave’s will be most helpful. I have spent some time adding some pictures of my own build into my album this afternoon…