Hull Shape, Speed and Trim Tabs

[Philip Stone]

I'm a proud new boat owner, that is I am newly an owner of an old boat, my first. My beauty is a 18ft welded aluminum custom built boat. Must have been originally designed and built for work here on British Columbia's coast. It was fitted with an inboard then but now in its new life as a family cruiser it has a retrofitted transom and a Honda BF 50 for power.<br /><br />My questions relate to the hull design/shape and how I should expect she will handle. I get about 12-15 knots but I can't quite get her on plane. She doesn't porpoise but she rides with the bow up and with a fine rooster tail off the stern. Also I'm not acheiving the full rpm at full throttle (I get 4,750 to 5,000 rpm and Honda says 5,750-6,000 rpm). So I know enough to see there's room for improvement. I've had a few motor trim tips in another forum I'll try. <br /><br />Pictures can be seen here:<br /> http://www.quadraisland.ca/naia/ <br /><br />Any feedback on these questions would be appreciated:<br /><br />-does the hull fit the 'displacement' hull category? What tells that one way or another?<br /><br />-the 50 hp picks her up okay but not quite on a plane, should I pursue trim tabs to get her up out of the water or run her slower and closer to the water?<br /><br />-will a different prop better suited to the boat change performance so much as to try that before trim tabs? I don't know exactly what prop is on there now, it is the stock unit that came with the motor.<br /><br />-I've been reading a lot on this forum about Smart Tabs, does she look like a candidate for those?<br /><br />Thanks in advance.

 

[CTD]

Re: Hull Shape, Speed and Trim Tabs

Wow, that is custom isn't it. Looks like a planing hull but looks a little heavy. Also to me looks like the motor might be a little low so that fin might be dragging more than helping. Someone here with the knowledge will let you know. I think tabs would help. How much extra weigh are you carrying (pass etc)

 

[Philip Stone]

Re: Hull Shape, Speed and Trim Tabs

Hi CTD, yup, custom is the word. Others have pointed out the motor might be a bit low but it isn't quite as low as it looks in those photos, the angle I took the pictures exaggerates it. Eyeballing it the cavitation plate and fin are bang on the low point in the hull. However when running with the bow up of course the fin is going to be lower than the hull in relation to the water surface.<br /><br />As far as weight goes that varies from just me, fuel for day and fishing gear (300lb) to: wife, kids, food and fuel for w/e and kayaks (~700lb).

 

[KCook]

Re: Hull Shape, Speed and Trim Tabs

I suspect the 50 simply isn't enough motor to achieve full plane. Enjoy it for what it does, or trade for something else. Very nice photos, BTW <br /><br />Kelly Cook

 

[Jdeagro]

Re: Hull Shape, Speed and Trim Tabs

Alpine:<br /><br />Two weeks ago I did soome boat testing and demonstrations. One was a 1984 Bayliner Ciera with a 1984 85 HP Force OB. We put five big guys in the boat (about 1200 Lb.) when the boat only seats four. The boat had a closed bow.<br /><br />With out the Smart Tabs the boat would not plane under any circumstances with this load. With the Smart Tabs it planed easily in about 8 seconds. Not a racer but ran nicely at 33 MPH (GPS).<br /><br />You have a planing hull and it should plane easily with trim tabs. I would guess that the top speed should be about 27 MPH+.<br /><br />Once on plane you will need to trim the motor up to get your best speed. You will also need to remove the Hydrofoil if you iinstall the Smart Tabs.<br /><br />Use the ST1290-60 and adjsut the pressure setting to the lowest point ( not the angle - just the pressure).

 

[Philip Stone]

Re: Hull Shape, Speed and Trim Tabs

Thanks nautiJohn,<br /><br />I was hoping to hear from you as I have read many of your posts on the subject of trim. If I hadn't once had it up on a plane (first day with the lightest load I've had in since admitedly) I would accept what Kelly says but given that I have seen it plane I want that option for the light load days. Smart Tabs seem an affordable option and my local chandler who I spoke to this morning approves of the product you'll be happy to hear.<br /><br />I was a little concerned there isn't enough room on the port side with the kicker bracket but I understand the actuator is 11" which gives me room to spare. The darn zinc I just had welded on the other day too and I might have to chop that off and move it a little to get the tab hinge mounted, or at least take a bite out of the mounting tab on the zinc to make room. Not a biggy I guess.<br /><br />The prop guy in town is advising changing to an 11pitch prop instead of 13 which is on now.

 

[Jdeagro]

Re: Hull Shape, Speed and Trim Tabs

Alpine;<br /><br />Do not change props until the boat hull is optimized ( ie trim tabs). Changing to a lower pitch will only cost you more money in fuel if it is not necessary. It will be like running arround town in 2nd gear. <br /><br />The Smart Tabs will give you the ability to plane easier and more top end in terms of both RPMs and Speed. Trim Tabs first, then prop if necessary to the best performance.<br /><br />As I said to someone else: if you were running 300 RPMS below the recommended range, and you were draging an anchor,would you change the prop to get into the range first, or pull the anchor in first? Make the hull as efficient as possible first!

 

[Philip Stone]

Re: Hull Shape, Speed and Trim Tabs

Gotcha! Thanks, tabs first it is.... as it happens my local dealer has the model you recommend. Standby for performance report...<br /><br />By the way your explanations and analogies are excellent.

 

[JasonJ]

Re: Hull Shape, Speed and Trim Tabs

It may not be a bad idea to weigh that boat. I am sure trim tabs will help, but it is very likely that there is just not enough power. 50 horses on an average aluminum 18 footer would give okay performance, but your rig looks like it has a lot more weight to it than the average aluminum 18 footer.

 

[Boatist]

Re: Hull Shape, Speed and Trim Tabs

I also feel you do not have enough power. Make sure your motor is running on all cylinders. If it is I think you just need to add power. Also older Aluminum boats with hull shapes simular to yours had a problem with forming hooks in the hull. Make sure the midship the hull is flat with on hooks. Adding trim tabs is not going to take the boat form 15 Knots to 27MPH +.

 

[walleyehed]

Re: Hull Shape, Speed and Trim Tabs

I think it'll work... <br />15kts. is about 17.5MPH, and if it will plane out, I don't see a gain of 8-10MPH out of the question once on top...<br />This ain't nautijohns first trip on the ship.......

 

[Jdeagro]

Re: Hull Shape, Speed and Trim Tabs

Alpine & others;<br /><br />WE guarantee the performance improvement, and I promise that we do not make claims just to sell product. It is unfortunate that some companies do - it makes all of us suspect!<br /><br />I think this boat will perform substantialy better with the tabs because of the experience we have had in the European countries. They use much less HP on their boats becaus the gas is $2.00 per Ltr., or nearly four times our cost. <br /><br />I recently received an e-mail from our distributor in Norway who said that Zodiac tested a 9ft inflatable with a 3.5 HP two cycle OB. As expected it woould not plane with this little motor and top speed was about 5 to 6 MPH. After installing our Smart Tabs he said it planed off (one person ) and got up to 14 kts. I questioned him on the HP and he confirmed that it was 3.5 HP.<br /><br />To that point I had never seen or tested anything less than a 6HP. <br /><br />It seems that when you controll the hull you can change the performance significantly.<br /><br />In this country we are spoiled ( I personaly like being spoiled), our gas, and equipment is much less, so we tend to be HP hogs. <br /><br />We have tested 17' aluminum Bass Boats with 50 HP and 75 HP. We made the 50 HP out run the 75 HP in accelleration to plane and to 30 MPH, and the top speed (one person onboard)was within 2 MPH ( 38 and 40 MPH respectively). We installed Smart Tabs on the 50HP. No Prop change and both boats were new. The 75 does not have that much more low end torque, and although the HP builds at the top end, the porpoising kept the 75 HP from running smooth unless the motor was trimmed down. This was enough to to limit the top end.

 

[Philip Stone]

Re: Hull Shape, Speed and Trim Tabs

Thanks guys. I realize that 50 hp is not a lot of power for my boat ( and nautiJohn you hit the mark about the cost of gas, that was a big factor in my choice to go with this size motor) but as I mentioned I have already had it up on a plane with a very light load. As that load is unlikely to be repeated I just need a bit of a boost to compensate for the additional weight I'm needing to carry. The hull has no hook and is a well constructed 'beefy' craft. <br /><br />The 50hp is brand new and has I haven't even realized the full manufacturers max rpm so I'm confident it is plenty of power to plane the boat with proper trim.<br /><br />I've talked with a local boat builder EagleCraft. They know aluminum boats http://www.eaglecraft.bc.ca the main guy there strongly suggested tabs and felt they would not only get her up on a plane with the 50hp but help improve transverse stability which could be an issue in a strong crosswind due to the high wheelhouse.<br /><br />As it turns out I have ordered a Bennett setup. No offence to the Smart Tabs which I seriously considered. It was my interest in learning a lot more about piloting more complex vessels in the future and that they were within my budget that helped me make my choice.<br /><br />I'll hopefully have the tabs installed and be on the water on the weekend so I'll report back then. <br /><br />Thanks for all the advice.

 

[Jdeagro]

Re: Hull Shape, Speed and Trim Tabs

Alpine;<br /><br />One reason you do not see helm controlled trim tabs on smaller boats is that they can be mis-used and cause serious handling issues. What i am refering to is the acidental deployment on one tabs while the boat is on plane and cruising. A boat of this size is rather light in Tab Terms, and it can be thrown arround easily. Most manufacturers consider this to be a potential liability issue on boats of this size.<br /><br />I am not saying that you will not get good results, but I am saying that you should practice and not let anyone who is unfamiliar with trim tabs opperate the tabs and/or boat without good instruction.

 

[walleyehed]

Re: Hull Shape, Speed and Trim Tabs

---Without VERY GOOD instruction.......

 

[JasonJ]

Re: Hull Shape, Speed and Trim Tabs

Definately take some good readings with GPS, time to plane, time to max speed before and after, and post so we can see how big of a difference has been made (make sure load weight and distribution is the same so the comparison will be accurate). It does make a big difference that you are running a new motor that has not completed breakin and you have not ran it to WOT. You may have better performance just waiting to be used once that motor is broke in. You also need to make sure you are acheiving near the upper end of your WOT recommendation (after the engine is broken in)on the engine with a light load, you may not have the right prop on her. Good luck and have fun with the new rig...

 

[Tabman]

Re: Hull Shape, Speed and Trim Tabs

I am usually pretty diplomatic, but John's statement regarding helm controlled Trim Tabs on Small Boats is just plain wrong. We have been manufacturing Trim Tabs since 1960. The speed at which our Trim Tabs operate is chosen for the best reaction without the danger of sudden changes in attitude if they are operated by mistake. Properly sized and installed Trim Tabs will not pose a problem to an operator on any size boat. Our Trim Tabs have been installed on boats as small as 13 feet and are regularly installed on boats 15, 16 and 17 feet long. Here in Florida you will be hard pressed to find a small, light Flats boat in the 16 foot range without Trim Tabs installed. <br /><br />We have tens of thousands of Trim Tab sets on boats in the size range John is referring to. We proved Trim Tabs for many welded aluminum boat manufactures and for sizes of boats like the poster has. We have manufactured over one million sets of Trim Tabs and we know what the liability issues are. Just like using any control on a boat, throttle, trim and tilt or Trim Tabs, prudence is always required of the operator. We have never had an issue using helm control trim tabs on that type of boat. <br /><br />The owner of that boat may not choose to install helm controlled Trim Tabs, he may choose a foil, Smart Tabs, or helm controlled Tabs, but let’s let him make that choice based on facts.<br /><br />I can understand John trying to define a niche for his product, but I have to take a stand when someone makes an unfounded claim.<br /><br />Sincerely, <br /><br />Tom McGow<br />Bennett Marine

 

[Philip Stone]

Re: Hull Shape, Speed and Trim Tabs

Ah the debate heats up. Well to keep you fully in the loop it's the M120 setup I've got, it was suggested to me to go as large as 12x24 by the Eaglecraft folk but I just don't have room for that span a tab on the stern so I figure the largest sport tab should be good.

 

[Solittle]

Re: Hull Shape, Speed and Trim Tabs

Sidestepping the tabs issue I agree with the guys about raising the motor on the transom and getting the rpms up by dropping the pitch of the prop. Also from the pictures it looks like you have the motor trimmed way to far down which will push the bow down. I would have addressed these issues befor I bought the tabs but that is a moot point for you.<br /><br />When you get it sorted it out I think you will be quite pleased with the performance. I think it is a neat looking rig & will certainly generate a bunch of questions from others as you tool around.

 

[Jdeagro]

Re: Hull Shape, Speed and Trim Tabs

Tom;<br /><br />Let me be clear on my statements regarding the use of Helm controlled trim tabs on smaller boats.<br />Proportionately, smaller, lighter, faster boats react more quickly to any change in trim, weight / load changes, outside wind and water force, speed variations, and even trim tabs adjustments. I suspect that this is the very reason your trim tabs are designed to react slowly. Obviously a very good design feature!<br />Not all of the trim tabs (helm controlled) manufacturers function in the same manner, and therefor, in my opinion, run a higher risk of improper control.<br /><br />What I was trying to say is that there is a learning curve and lack of practice, experience, and instruction can lead to uncomfortable situations. <br /><br />Apparently my statements seamed to target your company, and for that I apologize. <br /><br />If there is a choice in helm controlled trim tabs, based on reliability, quality, and customer support Bennett gets my vote from a personal view point. However, for boats 20' or under I do not believe they are the most appropriate choice.