Poor mans trim tab question (see image)

[Moody Blue]

I am trying to correct a slight performance issue on a 14ft whaler style boat with a 40hp Mariner OB. Hole shot is great but have to keep motor trimmed IN all the way or boat porposes once on plane. Transom angle is only 8 degrees and motor has a fin on it. Without the fin the boat porposes ALL the time. Top speed is about 33MPH but it won't plane below 25MPH as the stern slowly sinks into the water, the bow comes up and the speed drops way down. I know the answer will be smart tabs but I really can't afford them right now. I was thinking of installing two fixed tabs about 4"x10" at the outer most points of the stern. I have some 1/8" S/S sheet laying around I can use. I have included a drawing of the transom shape. The bow is a modified V shape. What are your thoughts? Will this help or am I wasting my time?<br /> Transom drawing <br />Thanks.

 

[Moody Blue]

Re: Poor mans trim tab question (see image)

Wanted to add that the motor is mounted with the cavitation plate even with the bottom of the hull. Hull is in perfect condition ie no hooks etc. Prop is 10 1/8" dia cupped aluminum. Not sure of pitch. No numbers anywhere on the prop. No tach installed yet so can't tell RPMs. Sorry for the verbage but wanted to add as much detail as possible. Thanks again.

 

[bayman]

Re: Poor mans trim tab question (see image)

RodC, I don't know if the Nauticus trim tabs are within the price range you would like. Because of their responsiveness I would recommend that you look at them though:<br /> http://www.iboats.com/mall/index.cgi?prod_id=24438

 

[chuckz]

Re: Poor mans trim tab question (see image)

If you can't plane below 25mph something is wrong. That's a light hull and you have plenty of power. Try moving weight forward. It sounds like in your case a fin or tabs will be masking the problem. Solve the problem first, then add the fins or tabs.

 

[bayman]

Re: Poor mans trim tab question (see image)

Have you looked at the bottom of the hull? Does everything look okay? No barnacles, damaged parts of the hull?

 

[Jdeagro]

Re: Poor mans trim tab question (see image)

RodC;<br /><br />The problem with adding fixed trim tabs to the hull is tha this constitutes a permanent change in the hull design. In order to eliminate the porpoising you will need to deploy the tabs at an angle sufficient to lift the stern and keep the bow from popping up. If for example you cruise at 27 MPH the tab adjustment needs to be set for that speed (deployment angle). However, speeds above 27 may show signs of excess stern lift and resulting bow down. This will cause bow steering and handling issues, and less speed at the top end. In other words, you will need to pick what suites your needs as well as possible, understanding that what ever it is will be a compromise. These types of tabs are on the market now.<br /><br />The larger you make the tab the more effective it will be - obvious - and the low end improvement (getting on plane and staying there at lower speeds) will be more responsive. However, again we have a compromise as the larger plate will also effect the cruising and top speeds more. <br />Improving your hole shot will take more deployment angle than eliminating the porpoising at cruising speeds. Therefor you will need to pick a median point. <br />Each and every time you load the boat differently, will also change the handling and need for more or less "tabs". <br /><br />If you do not have the money (About $140.00) at this point for Smart Tabs, then try the fixed tab idea, but make sure you can adjust them, and expect to make a few trial and error runs to fiind the best compromise.<br /><br />As an active system the self adjusting feature of Smart Tabs is what makes them so effective.<br /><br />One post suggests that you look at boat balance, and so would I suggest this, as what you are telling us sounds as though the stern weight is very high.

 

[mellowyellow]

Re: Poor mans trim tab question (see image)

try weight distribution first... a hook in the<br />bottom of the hull is remote on a whaler IMO.

 

[Dave Abrahamson]

Re: Poor mans trim tab question (see image)

Originally posted by RodC:<br /> I know the answer will be smart tabs but I really can't afford them right now.

And yet 2 out of 4 responders told him to check out "Smart tabs" anyway <br />(OK nautijohn doesn't count....he always recommends them. )

 

[shep70057]

Re: Poor mans trim tab question (see image)

I would believe that there is more weight in the bow. Distribute most of the weight to the rear.

 

[umblecumbuz]

Re: Poor mans trim tab question (see image)

There have been several previous posts with photographs of fixed tabs - home made angle screwed to the transom - and each owner reported big handling improvements.<br /><br />If you can make a pair with some kind of adjusting rod, then give it a shot - why not?<br /><br />Tabs are so effective anyway that the worst you'll ever need to do is take them off again. What have you got to lose?

 

[BillP]

Re: Poor mans trim tab question (see image)

You can do adjustable homemade tabs with 1/4 thick alum sheet. Attach them to the hull with continuous hinge. Use common turnbuckles for adjustment (2 for each tab)...one end to the tab and the other to the transom. Eyebolts (or pad eyes) can be used to connect turnbuckles to the tab and boat.

 

[Realgun]

Re: Poor mans trim tab question (see image)

What size do I need for a 17.5 foot bayliner?

 

[Jdeagro]

Re: Poor mans trim tab question (see image)

We did some testing on nearly the same boat and engine comgood condition and ran fairly well given that it had been sitting for a year. With five heavy men in the boat (1100 lbs.) it would not plane. We installed 12" x 9" Tabs and it planed in about 7 seconds. The top speed with the five people (1100) was about 32 mph, and with two people or (450 lbs) it was 36 mph.using a GPS. I would not use any thing less as the boat can use all of the help it can get with that motor.

 

[roscoe]

Re: Poor mans trim tab question (see image)

Have you tried running it without the fin on there?<br />Sure sounds like something is wrong. Could be the wrong fin, on the wrong motor, at the wrong height.

 

[Moody Blue]

Re: Poor mans trim tab question (see image)

OK. No question the stern is heavy on this boat. The 40HP weighs about 50Ib's more than the 25 its rated for and IMO the console is placed too far to the rear thus more weight astern. I've moved the battery and fuel tank forward as much as possible but it makes only marginal improvement. <br />BillP - that is exactly what I had in mind. Have you tried this yourself ?<br />NautiJohn - I wish I could afford those tabs but in reality they would be well over $200 by the time I factor in shipping, taxes, and the CDN$. I'll try the cheapo approach first and keep my fingers crossed. In your second post what boat are you refering to ? Are you saying the 12x9 tabs are for the Bayliner or my boat ?<br />Roscoe - took the fin off once and couldn't plane at all. Boat launches from hole shot but porposes wildly. Tried powering through it but just got worse. The fin makes the boat driveable .... but just.

 

[umblecumbuz]

Re: Poor mans trim tab question (see image)

Depending on your fin type, have you tried packing the leading edge so that the fin is no longer parallel to the a/c plate - but'slopes' slightly upwards?<br /><br />I have seen that done, and (so it was claimed)it had the effect of allowing the motor to be trimmed our a little while the angle of the fin still gave stern lift. Suppopsedly gave more forward drive with lift at the same time.<br /><br />You pays your money . . . . .

 

[Moody Blue]

Re: Poor mans trim tab question (see image)

Stillfishing I hadn't thought of that. Makes sense that changing the angle of the fin would alter to trim angle. Think I will still try the home-made trim tabs first. I like a challenge. Me against the machine (or physics as it were). Besides I have some S/S plate kicking around that I didn't know what to do with.

 

[BillP]

Re: Poor mans trim tab question (see image)

Originally posted by RodC:<br /> OK. No question the stern is heavy on this boat. The 40HP weighs about 50Ib's more than the 25 its rated for and IMO the console is placed too far to the rear thus more weight astern. I've moved the battery and fuel tank forward as much as possible but it makes only marginal improvement. <br />BillP - that is exactly what I had in mind. Have you tried this yourself ?<br />NautiJohn - I wish I could afford those tabs but in reality they would be well over $200 by the time I factor in shipping, taxes, and the CDN$. I'll try the cheapo approach first and keep my fingers crossed. In your second post what boat are you refering to ? Are you saying the 12x9 tabs are for the Bayliner or my boat ?<br />Roscoe - took the fin off once and couldn't plane at all. Boat launches from hole shot but porposes wildly. Tried powering through it but just got worse. The fin makes the boat driveable .... but just.

Yes, on a Stephens SK with 110 Merc and another friends ski boat that I can't remember the model...that was back in the 60s. You could probably do this cheap ($25-30) for a trial by using large galv gate hinges, pad eyes and galv turnbuckles. Back then most people made plywood "fixed" tabs for skiffs.

 

[Moody Blue]

Re: Poor mans trim tab question (see image)

Thanks for the replies. Not sure of the size to make them for this size boat/motor. Any thoughts ?

 

[Jdeagro]

Re: Poor mans trim tab question (see image)

RodC;<br /><br />I understand! The plates should be 9'wide X 8" deep. The Bayliner used the 12" x 9" plates.<br /><br />No matter what you do with anything fixed you will need to do some trial and error testing to get it as close as possible to what you want. I would recommed rigging fixed trim paltes as opposed to adjsuting the angle of the foil. I think you will run into more handling issues with the foil adjsutments.