Originally Posted by
giddyup58
Hi all.
We have been using the Mustang a bit over the past few weeks, and decided that I have to stop it porpoising as it is tiring and uncomfortable.
At high speed, the cav plate was only just visible, and small amounts of trim would cause the dreaded porpoising. I could get away with about 25% +ve trim.
So I had a chat with ol' mate Cyril, and we decided to come up 1 more hole, fill the front water tank (100 litres) and 2/3 fill the fuel - 210 litres.
Took it for a run yesterday on the river. I can't believe it, but the porpoising has totally disappeared. I can trim the motor out so far that it throws a rooster tail behind the boat, with no porpoising. I can trim +50% with no slip and with NO ill effects, and top speed has climbed a bit more to 46.2kts. (with tide and wind...)
3,500rpm = 25kts @ 24lph = 1.91km/l
4,000rpm = 27kts @ 30lph = 1.66km/l
4,500rpm = 34kts @ 40lph = 1.57km/l
5,000rpm = 36kts @ 50lph = 1.34km/l
5,500rpm = 40kts @ 67lph = 1.10km/l
6,000rpm = 44kts @ 95lph = 0.86km/l
6,100rpm = 46kts @ 112lph = 0.75km/l
The most economical speed is 3500rpm, but the most economical fun is at 4,500rpm. The boat really settles nicely at speeds over 30kts, and the biggest jump in efficiency seems to be between 4,000 and 4,500 - 7 knot increase for 500 rpm, as opposed to roughly 3-4 knots per 500rpm just about everywhere else.
And the beauty of that speed, is the hull is so aggressive, the speed can be maintained in quite heavy conditions...
Luckily, now I do not have to fit the permatrim.
As has been discussed many times on these boating forums, foils are best seen as a "bandaid" for an incorrect design/set-up.
Best practice is to change set up - engine height, weight distribution trim angles etc. to cure the porpoising and if that doesn't work, fit a foil.
Setting up this boat has been a chore.
You will recall that we had to try about 5 propellers before we settling on the genuine Suzuki 3x16x18.5 as the best prop, change the leg of the motor to an XL from an XXL, redesign the pod height and play with weight distribution to get to this point. And thankfully, no foil...
The boat would be horrible without the trim tabs, as there is so much torque generated by the motor and the V is so acute (24 degrees), that on flat calm water she always runs portside depressed. Which is a bit annoying, and which a foil may well fix, but I'm ok to put the port tab down a touch to level things out...
Now that it is finally sorted out, it's time to finisht the rest.
Hopefully, some stainless and upholstery work in the next couple of weeks, then I may get into finishing the teak work.