PaulGooch
Well-Known Member
10 knots is just a wrong speed for that boat. You need to be doing 6-7 knots, or planing.
Trim Tabs
- Thread starter David of Essex
- Start date
10 knots is just a wrong speed for that boat. You need to be doing 6-7 knots, or planing.
thejonesey
Well-Known Member
I agree about the speed not being ideal but she does not rise up onto the plane, ie over the hump and flatten out, making me think she is semi-displacement. Even at full tilt she runs very nose up. How can i tell what type of hull she is as I have seen it described, semi-displacement and semi planing?
MarieK
Well-Known Member
What speed are you getting when she is flat out?
benjenbav
Well-Known Member
Apologies in advance for fred drift but I made a discovery a couple of weeks ago in connection with trim tabs on my semi-d vessel which made me both blush and laugh.
What I discovered was that you need to press and hold down the buttons (bennett), not just click them like a keyboard button.
I've only had this boat for three years...
And, yes, I reckon I did get an extra knot and that the angle of attack was improved, but the difference wasn't mega.
My last boat reacted much faster to a small adjustment via the tabs, in fact needed trimming fairly actively.
What I discovered was that you need to press and hold down the buttons (bennett), not just click them like a keyboard button.
I've only had this boat for three years...
And, yes, I reckon I did get an extra knot and that the angle of attack was improved, but the difference wasn't mega.
My last boat reacted much faster to a small adjustment via the tabs, in fact needed trimming fairly actively.
ian38_39
Well-Known Member
Know someone who runs an Antares 9, similar boat and he cruises everywhere at about 14 knots using no fuel at all with a single D4, so doubt you could call them a true planning hull, if it was you would be squarely on the hump and using tons.
If I am not mistaken they also have a decent run of keel which helps with the leaning into the wind issue.
He has tabs but says he very rarely touches them as they make no difference to the ride.
If I am not mistaken they also have a decent run of keel which helps with the leaning into the wind issue.
He has tabs but says he very rarely touches them as they make no difference to the ride.
PaulGooch
Well-Known Member
I'm pretty sure you hull is semi displacement (semi planing means the same thing), rather than full planing.
At 10 knots you're not on the plane, but you are exceeding the displacement speed. Your bow will be high, the stern will be in a hole and the water at the back will look like a cauldron. Now, ease off the throttle a little at a time, as you do so your speed will drop slowly, the engine RPM will drop faster. When you reach maximum "sensible" displacement speed, the water at the back will stop churning, the bow will lower and the boat will level out, the engine RPM will be hugely lower, but the speed will only be slightly lower and everything will be much quieter, your fuel bill will be vastly lower. When on the plane, the bow will sit about the same as in the OP's pic.
Divemaster1
Well-Known Member
First of all, looking at your picture, she looks like running at an OK angle, but that can of course be deceiving.
Now on to a couple of "IF's"....
1) If she is running on shaft(s) that are angled
2) If pushing bow down does not increase friction significantly (always a risk)
... then theoretically you could achieve a slightly higher speed for two reasons..
a) you will present a slightly larger waterline length, which theoretically should mean higher hull speed.
b) the propellor will direct waterflow more directly astern (rather than downwards), should theortically mean greater propulsion efficiency, and thus higher speed (or lower fuel consumption for given speed).
Now there are a couple of issues you need to be aware of...
1) You ought to be able of running as if the flaps were not there.. (see 2 below)
2) In following seas your possilbe lift surface (for the sea) is greater, so she may change behaviour in following seas. 1 above will minimise this, but will not eliminate the risk.
Now on to a couple of "IF's"....
1) If she is running on shaft(s) that are angled
2) If pushing bow down does not increase friction significantly (always a risk)
... then theoretically you could achieve a slightly higher speed for two reasons..
a) you will present a slightly larger waterline length, which theoretically should mean higher hull speed.
b) the propellor will direct waterflow more directly astern (rather than downwards), should theortically mean greater propulsion efficiency, and thus higher speed (or lower fuel consumption for given speed).
Now there are a couple of issues you need to be aware of...
1) You ought to be able of running as if the flaps were not there.. (see 2 below)
2) In following seas your possilbe lift surface (for the sea) is greater, so she may change behaviour in following seas. 1 above will minimise this, but will not eliminate the risk.
thejonesey
Well-Known Member
Mariek -Wot is 25 knots (with clean bottom) fast cruise is 18-20 at 3000rpm. Paul -thanks for explanation about wake etc. I come from a planing hull background so this semi-d is all a bit new! It sounds like I won't gain much from tabs. Fuel economy is good and she feels planted in all sea types. Only option is to give it more welly!
Thanks
Thanks
PaulGooch
Well-Known Member
petersboats
Member
We have just fitted a set to a twin engine Rodman. Made a big differance to performance.