trim tabs and hull speed

[jaguarmk2]

I never knew till last week that a boat has a hull speed goverened by its water length, i have a Ford 27 that travels slightly above hull speed, i have been told fitting trim tabs will make a huge difference but i have also been told to save my dosh who is correct?

 

[Deleted User YDKXO]

If your Fjord cannot exceed it's displacement speed ie. hull speed, then trim tabs will make little or no difference to it's speed. On the other hand, if it can exceed it's hull speed ie. plane, then trim tabs may well increase boat speed at slower planing speeds. Lowering the trim tabs have the effect of lifting the stern of the boat a little and assisting the boat to plane

 

[jaguarmk2]

how do i know before i purchase them? the bow lifts a fair bit when i apply power the boat has an aft cabin so the stern is quite heavy the previous owner had filled the bow with sand but i removed it when fitting out the cabin after some repairs to the floor, it is worth putting sand back in the bow(increasing the overall weight as well as the bow weight) or is there any other info i could give the boat engine is 106hp

 

[Deleted User YDKXO]

Is the boat supposed to plane or not? 106hp sounds a bit marginal for a planing 27 footer. Weighting the bow could help the boat to plane but, once planing, you don't want the bow to be too heavy otherwise there is the possibility of stuffing the bow into a wave. Fitting trim tabs is a better solution
You could try putting 3 or 4 crew into the forward cabin to simulate the effect of weighting the bow and seeing if the boat will get on the plane but ensure that the hull and sterngear is clean and the engine is in good order first for best results. Apart from the usual Bennett tabs have a look at Volvo QL tabs as well

 

[jaguarmk2]

the hull is a semi displacement so probably wont plane, but the ql tabs might let it ride level and push the bow through the water rather than pushing it with the hull, it would also let the prop push the boat along rather than upwards, is my theory going to let the speed increase or is this wishfull thinking?

 

[BrendanS]

Wishful thinking I suspect. The trim tabs would need to be enormous to really extend the hull length and change the hull speed. They can have other effects, but extending the hull length and speed is not one which will typically work. Low speed effects as Deleted User says are possible

 

[colinmi]

The boat was not built to plane. It was built to run at displacement speeds with a bit more oomph to get a knot or two extra if needed, on the odd occasion.

There is a simple calc for design speed at dispalcement speed. But good rule of thumb is, if the bow is starting to rise, then the *rse is digging a hole and you are burning serious qtys of fuel for that extra knot or two.

My boat cruises economically at just under 8 knots but it can make 10. Fuel consumption is doubled.

Bit of a ramble. Had a couple of whiskies, bottom line is dont waste your money on tabs. Slow down and enjoy the scenery.

cheers

Colin

 

[Deleted User YDKXO]

If it will exceed hull speed then it planes. Semi displacement is just a word for a boat that does'nt lift out of the water to plane as much as a full planing hull. Trim tabs can be very beneficial for semi d hulls as they help the hull to lift and hence increase speed
If your boat can exceed it's displacement hull speed = 1.34 x sq root waterline length( say 23ft) = 6.4 knots by, say, 2-3 knots, then IMHO it's starting to plane and trim tabs could well help you get another 2 knots or more. What speed will it do now flat out?

 

[ebbtide]

My 28ft semi-displacement angler with 120HP Perkins has QL trim tabs - small, but they make a significant difference - you can actually see the bow drop and speed increase as you adjust, and fuel consumption reduces too.
Wouldn't be without them.

 

[Andrew_Fanner]

My, now sold, Fjord 27 did have tabs and did plane, the tabs being useful for same. But rather more grunt from the engine. Don't forget that even at lower speeds tabs mau be useful to adjust the trim from side to side, if you have a lot of junk in the hold or a fat knacker like me at the wheel and everyone on the same side saying "look there's a seal" or what have you.

 

[tonybarley]

I have taken advice on a few issues regarding trimtabs with Bennett Trimtabs in Florida.
My contact was a chap called Mr Thomas.
Email him via their website - www.bennetttrimtabs.com
One point; I accept that they are running a sales operation, however, I have found their advice to be totally objective.

 

[brianrunyard]

Have a look at this site Tabmans Forum .
It's got to be about the most helpful and best after sales service of all.
He gave me very good advice when I was considering fitting Trim Tabs, I've now fitted them and the results are as good as he predicted.