Plane sailing?

[Innes_Ker]

Please help out a raggie. Had a debate last week as to how much putting extra weight on board might affect a power boat's ability to get on the plane. Example discussed was a 25ft boat of about 2,500kg and 30 knot top speed, under ideal conditions. If one then loaded up with say 4 to 6 adults, food & drink for the weekend, full fuel tank (350 litrs) and water tank, performance would obviously suffer. But by how much?? Top speed down by 10%, 25%, 50%? Might it not even get onto the plane?
Would it matter where mother in law was seated?
Would a dirty hull make much difference?

What would you guys expect?



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[BustinAround]

Dunno about the % loss in performance, but a dirty hull makes a hell of a difference!

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[neale]

You would need an awful lot of weight to stop the boat getting on the plane, fuel consumption would take a bashing though. Too much weight aft could stop it but then the weight could be moved forward to help.

I have had enough fouling in just 2 months to stop the boat getting on the plane.

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[jhr]

Had a practical demonstration of this yesterday. My boat is 22ft and 2,000-ish kg. We had 4 adults, 2 biggish teenagers and 2 small kids on board (RCD load limit is 800kg/8 people, before anyone asks) plus 3/4 full petrol tank and full water tank.

I normally cruise at about 3,500 rpm and expect to get about 22-23 knots through the water, give or take, with myself, SWMBO and our 2 small kids (as above). The main difference yesterday wasn't in the speed attained, but the fact that it was a bit of a struggle to get on the plane. This was partly about distribution of weight, because things improved when I moved one of the, ahem, larger adults forward, but I also suspect that the increased load, which must have been in the region of an extra 200kg over our normal level, had a bearing. However, once we were on the plane it was almost business as usual, though I suspect I would have had fun getting near normal maximum revs (usually about 4,500) which would have reduced top end speed.

Yes, dirty hull and/or props make a significant difference.

Positioning of the Mother in Law is critical. I normally trail her from the stern, but you need a good, stout warp to do this. /forums/images/icons/smile.gif

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[jfm]

I would normally expect a dirty hull to cost about say 3-4 knots off top speed, but last week I realised it can be much more if speed is higher, and also it might affect different boats differently.

I have generally found that a 26 knot boat can drop to say 22 ish knots with a dirty bottom. But our new boat was down to 21kts 3 weeks ago, having collected 6 months of barnacle/weed, and having been 34 knots when new in March 04. 34 to 21 knots is a massive reduction, so we got a diver to scrub the bottom and props for 2 hours, and this got it up to 25 knots. As this obviously wasn't good enough, last week we had it lifted out, cleaned properly and new antifoul, and when dropped back in it was right up to 34 knots again. So it seems that on some boats at least, particulary 30knot+ boats that still have a lot of wetted area at that speed (as is the case with all biggish mobos), a moderately dirty bottom can cost you perhaps 8 or 9 knots

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[andy_wilson]

Top speed is likely to be least affected, maybe 10% reduction tops.

Weight distribution may become critical for rising onto plane however, when everyone is sat at the back more trim or tabs may be required or in extremis, move someone forward to climb the hump.

Dirty hull and / or prop(s) will probably matter more, affecting performance by 10-20%.

Always ensure bottom is cleaned thoroughly by Mother-in-Law (who are good at cleaning things generally) before departure. If you are still unsure, forget to take her back on board when you set off.

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