Top speed 215HO drag on cooling plate

[gRobi]

Hi guys,

I'm trying to increase my top speed a little bit. At the moment, I'm looking at the back of my cooling plates. There's a odd looking peace at the end of the cooling plates. I think I remember someone wrote it will work like a little trimm plate to stabilise, but I really think all it will be doing is increase drag. Any enlighting thoughts, before I'll remove them for testing?

 

[Luc Lafreniere]

That's what if does by design. Increase drag. It brings the bow down to avoid porpoising (bouncing). Yes it does rob you of some speed.... But I'm not sure porpoising is worth the speed gain. Porpoising happens at different speeds for different boats and depending on weight distribution. More weight in the front reduces or stops it.

So if you do remove them, you may need to add weight in the front. Funny enough, someone didn't have these on their boat on Facebook and justed added them because it was proposing a lot.

 

[TeenGee]

If that is where the engine is cooled, I suspect the dam is there to slow the boundary layer passing over the cooling plate. It is possible that removing that dam will reduce the cooling capacity of the cooling plate and overheat the engine.

 

[Luc Lafreniere]

It isn't for cooling. 165 boats for example such as mine don't even have this. Including the 300HP engine. It's all about preventing porpoising/stabilizing only.

 

[gRobi]

Okay, then I wasn't mistaking to have read that. I really can't imagine, such tiny things can stop porpoising, but I will test it tomorrow, looking at the speed gain and porposing and then decide what to do. Wouldn't it be better to habe a small bend instead of having just the straight alloy for drag direction? I really think, this is not an ideal design at all ‍

Question is (for example) why not trim the venturi in the downposition to stop porposing instead of adding drag

Don't get me wrong. I'm not an expert in any way, I'm just wondering, why they do it this way.

 

[Luc Lafreniere]

I'm not a BRP engineer. So I can't comment on their choices. But from experience, the venturi can be pointed down. I've done it. There's a bushing already there that can allow you to do this. But in my 165, it didn't stop the porpoising. No noticeable difference. Essentially, the nozzle direction and available play isn't enough to make a difference.

My boat hull has a curve in it and no such trim pieces. And I can tell you it's also not enough to stop porpoising on the 165. I presume this is why they are adding those on boats.

You can also have trim tabs of course, but they conflict with the reverse on out jet boats. The best solution for this are so called "interceptors". Essentially a plate that can go up/down to add more or less drag. Volvo makes some. But yeah, what you have there is essential a small interceptor.

Some even add a bead of silicone on the trailing edge of the boat. Does the same thing and it takes very little to make a difference. But all at a cost of efficiency.

 

[gRobi]

Thank you very much!!
Interceptors is the keyword. Found some interesting stuff. Seems Scarab would have had to install an automatic system to have instant planning and top speed combined but decided to install fixed ones at cost of to speed.
I'll give it a try

 

[gRobi]

Tested it.

The plates are fixed with 1 screw and glue. No problem to get them off.
Gained 4km/h making 94km/h with Stage1, 2 people and full tank. No noticable effect while driving fast. Driving 15-30km/h, a little bow rise but I think I have noticed a clearer wave, that in fact will be a good thing for wakeboarding. At least for me, it will stay this way, as long as I don't find anything negative.

PS: venturi is in the downposition by default, so nothing to tweak here

 

[Luc Lafreniere]

Glad it's working out! Give it some time to make sure though. Sometimes a little weight shifting around... slightly different weather conditions, etc. But hopefully it will remain the way it's currently behaving.