Trim Tabs

[Poida]

I have a hartley designed inboard boat that has a max of 10 knots in calm conditions.

It is fitted with trim tabs, is that really necessary on a boat that slow or do you think it is just making up for bad weight distribution?

[marshmat]

Hmm..... powered tabs or fixed? If they're powered ones and you never use them, or try them and they do nothing.... it's added complexity that's probably unnecessary. 10 knots.... this thing's not planing there, is it? or even semiplaning?

[PAR]

More often then not, trim tabs are on a boat to solve a problem the owner has no understanding of fixing. 10 knots could mean semi displacement speed, but we don't know the length of your boat's waterline, to be sure. You are correct in the assuming that trim will likely fix the issue that brought on the tabs in the first place. Empty the boat of everything that doesn't need to be there, I mean everything, then take her for a spin. If ill effects are shown move the tank(s) around, battery(s) may some coolers full of water, just to find her sweet spot. Once you're there, balance the weights around this general location, stealing from the stern to pay the bow off in pounds of trim, etc. This will take an afternoon, but it will be time well spent.

[Poida]

Thanks guys
Fixed tabs, 6m (20) boat waterline probably around 5.5m. Fuel tank fixed at stern. Straight 6 cyl engine around midship. Also cabin up front so most passengers ride in the back half.

I want to move the steering forward and add another seat next to the wheel may solve problem.

But taking weights on board is a good idea to find the best distribution.

[rayk]

PAR is dead right.
Having a spring clean is a good start.
Re stow loose equipment (forward) on the boat next.
Moving batteries forward is next.
Tankage is best located around the sweet spot to eliminate trim changes.

Adding ballast is a good way to find the sweet spot, but as a permanent solution ballast is in the same category as permanently lowered trim tabs.Ballast is more weight to get up on the plane.
Trim the boat around the sweet spot for the loaded condition that the boat mostly operates in. Raise the trim tabs first to find it. Bye bye drag.

Powered trim tabs are useful for minor trim changes related to variable load carrying conditions. Like fuel consumption from large fuel tanks located forward. Weight is gone from up front so a little down trim is no big deal.

Going fishing on your own, or with four mates from work, it is more convenient to operate powered trim tabs than rearrange the load (Hey, can you two guys go and sit in the cabin for a while...?). Letting the kids ride the bow or sit on the fore cabin helps too.
But putting the tabs down with a load in the stern is double work to get on the plane.

Have fun and a Merry Christmas.

[Poida]

Thanks Ryak

Par indicated that the builder of the boat put the tabs on to solve a problem that was created by the builder. I investigated along those lines and looked at the Hartley website. It appears to me that the builder has moved the steering back and made the cabin longer to accomodate a toilet. This would have moved weight backwards as well as the weight of the people that would have been located more forward. It looks like the trim tabs were added to compensate for the rear weight.

Removing them would also solve another problem I have with reversing, as the boat won't turn in reverse. Most likely because the tabs are in the water the pressure of water on them is holding it straight. This is mainly a pain when docking using the reverse turning to bring the boat along side a jetty.

Probably I'll have to wait until after Christmas to do the tests as my learned friends have described. Merry Christmas to you to mate.
Poida

[PAR]

Many straight shaft (inboard) configurations don't steer worth a damn in reverse. The tabs aren't the problem, if you remove them it still will steer like crap in reverse. I've owned boats that required careful planning when thinking about reverse, like trying to work with the prop walk, holding the helm hard over and "blipping" the throttle, etc. Some boat configurations just don't back down very well, which is particularly true of straight shaft installations.

At 10 knots, your boat is planning, though you're not going very fast. Displacement speed for your boat is about 5 and a half knots. You're doing almost twice that, meaning your speed length ratio is about 2.4, well into plane mode.

Starting with a naked boat, you can find the best balance for your reasonably simply, with shifting gear or weight. Another method would be to calculate the differences in weight centers, as a result of deviations from the original plans. This is a tedious calculation, requiring the masses of the boat (literally everything) in relation to the bow and LWL. You'll eventually end up with a vertical total and horizontal total which locates the center of mass for your boat (as it exists currently). This can be compared to the original plans, to see how far it's moved, so that adjustments can be made. Trust me, it much easier to shift some gear and or weight to find the balance.

Additional weight and deviations from original plans are the biggest performance killers of small craft. A cute little doghouse, on a small power cruiser, can stuff the bow down so bad, she'll never plane even with twice the recommended HP, but it sure looked good when the builder decided to change the plans. It's a common problem, but most can be fixed with trim adjustments, by moving, removing or possibly adding weight in key locations.