hull extension as trim tab?

Discussion in 'Boat Design' started by troppo1, Dec 18, 2011.

  1. troppo1
    Joined: Aug 2011
    Posts: 11
    Likes: 2, Points: 0, Legacy Rep: 35
    Location: holland

    troppo1 Junior Member

    I have a 5 ton 8mtr motorboat with a wl lenght 7.3 and am building a swim platform/hull extension that is 70cm long.
    If I follow the hull lines the stern would end up out of the water so I have to flatten the bottom to keep the stern in the water. Boat is only used on the rivers here in holland, ocassionally on the nth sea( if it is calm enough)
    I have found a picture of the same boat with the same extension but the hull line extends down 70mm? past level.
    I gather this is to work like a trim tab and push the boat up under way to stop it squatting.
    Any advice/ info on this.
    Is it better to keep the hull level under the water or make the bottom go down a bit to push the boat up? hull speed 6knots at 2200 rpm perkins 4-108.

    picture of the other boat:
    http://troppo1.mine.nu/photo/photo_...name=5a77656d706c6174206372797374616c2e6a7067
    picture of my boat as it is with wooden frame made to see how it looks and to order the steel cut to size
    http://troppo1.mine.nu/photo/photo_one.php?dir=766572686f656620382e33&name=44534346393534382e4a5047
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Use a straight edge, oriented parallel to the boat's centerline, to extend the hull shell aft, for the platform. Don't sweep up or down, just extend the bottom, just as it currently is, a bit longer, to accept the additional length. If this causes the aft end of the platform to clear the LWL, then you're getting a better "exit". Don't let the platform hang below the transom of the boat. I don't know what they are trying to do with that box/platform in the photos, but they are absolutely clueless in regard to the huge impact it will have on the boat's preformance envelop. Wedge some 2x4's against the bottom of the boat, paralleling the centerline, this is where the bottom will be, no lower, no higher, no different angles.
     
  3. troppo1
    Joined: Aug 2011
    Posts: 11
    Likes: 2, Points: 0, Legacy Rep: 35
    Location: holland

    troppo1 Junior Member

    Par, I asked here what the idea was and the answer was:

    Additional wl length= less diesel used at the same rpm and a bit more speed
    airtight compartment= extra bouyancy thus less squat
    keeping the stern in the water stops wave slap under the stern( bow wave from passing boats when at anchor. There are no motorboats in holland with a transom above water they are all built like this,( although this is the first I have seen that goes below level) all the sailboats do have. They also use the compartment for fuel or toilet waste( we cant empty into the water here, has to be pumped out) Have you any pictures of motorboats built with sailboat sterns in the states. In Aus they are also straight down and kept in the water.

    I was looking for 6knots at easy revs instead of the 5knots that all motor boats seem to do here, so i thought this was a 3 in 1 solution.
    As a sailor I have always had sugar scoops on my multi and the last mono, but this is the first motor boat.
    I also need to keep the exhaust pipe at least 50mm above the waterline at the back and the pipe is 50mm so I need at least 100mm at the stern above the waterline and would like to extend a boarding ladder under the water to make it easy to get out of the freezing water.
    what is the huge impact you mention on performance?
     
  4. rwatson
    Joined: Aug 2007
    Posts: 5,852
    Likes: 290, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    I'm no expert on hull extensions, but your boat doesn't look like it would benefit from any kind of 'scoop', or large trim tab - which is what the 'droop' (or Hook) on the wooden extension seems to be attempting.

    As I understand it, trim tabs are only good for high speed planing boats with high power to weight ratios, and would just be a real drag for a displacement hull like yours.

    I think that's why Par is saying to keep the extension parallel to the waterline.
     
  5. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    What I'm saying is extend the bottom just as if the lines were drawn that way, no attitude changes, no steps, no boxes, just a continuation of the hull's buttocks aft. It's unlikely they'll parallel the LWL and it wouldn't benefit anything if they did.

    Preventing squatting in a displacement hull form at 6 knots is a ridiculous concept, as envisioned with the contrivance I see in some of those photos. That box will only add drag and some buoyancy, but the boat will still squat. The additional LWL length will amount to about 1/4 of a knot improvement in the theoretical hull speed, which frankly isn't enough to warrant the hull extension in the first place.

    As RWatson has mentioned, tabs aren't going to do much (read add more drag) though you might get a 1/2 a degree bow down trim at full speed, which again isn't enough to get excited about.

    If you're traveling at displacement speeds, you'll want the stern clear, not in the water for best efficiency, so this preconceived notion as to the opposite, seems ill advised.

    Powerboats with an immersed transom are designed to motor at higher then the theoretical hull speed limit (semi plane) so they need this immersed area. It's important to compare apples against apples. At 6 knots, you are well below the theoretical hull speed limit, which for you should be very nearly 6.6 knots. At 6 knots you'll be traveling at a S/L ratio of 1.23 a full 9% less then the theoretical hull speed limit of 1.35. This is all without an extension. Lastly, I find it quite difficult to believe, that there are no true displacement power craft in your country. It doesn't have to have a sailboat stern, to provide a reasonable "exit" at displacement speeds, but dragging a hole, the size of the hull's displacement behind the boat, while underway isn't remotely close to being efficient, particularly if traveling at these speeds.
     
  6. troppo1
    Joined: Aug 2011
    Posts: 11
    Likes: 2, Points: 0, Legacy Rep: 35
    Location: holland

    troppo1 Junior Member

    Ok Par, I am begginning to get the idea. After I had seen the foto of the other boat and made the wooden frame I kinda had my doubts and thats the reason of this post. Over here they extend flat sections of the hull onder the water and dont close it in(continuation of the hull's buttocks aft), supposably for extra wl length or they build the whole thing in and use it for fuel or wc tank.
    check out the foto's:
    normal hull extension on a motorboat in holland
    http://troppo1.mine.nu/photo/photo_...57273636869705f303032325f30325f6a70672e6a7067
    and
    normal continuation of hull onder the water without enclosing it( i find this is a bad idea because it fills up with growth, waterbirds build their nests on it and so on) also a few more boats like it on the page.
    http://troppo1.mine.nu/photo/photo_...=57696e7465727374616c6c696e67253230322e6a7067

    and while looking I just found another one with the hull built lower than the transom, so maybe it happens more often here.
    http://troppo1.mine.nu/photo/photo_one.php?dir=68756c6c&name=626f6f7432312e6a7067

    Now my question is, is it better to leave the hull as it is and build a platform above the water or still extend the hull lines to they come clear of the water and build it in, as said above " If you're traveling at displacement speeds, you'll want the stern clear, not in the water for best efficiency" (if possable at 70cm) the 70 cm is is to keep the boat under 9mtr so that I dont have to pay extra for a berth( really expensive here over 9mtr), is there any benefit at all from building it in? My misses really likes to lay flat and sunbake on the swim platform and I want to carry the dinghy on it also. I always thought the addon platform above the waterline wasnt strong enough to withstand damage that happens a lot in the locks here.
    http://troppo1.mine.nu/photo/photo_...66f746f73&name=736c7569736a655f766f6c2e6a7067

    I like building things on boats, my last hull extension on my motorsailor before I bought the motorboat, but as said above it made no diff to performance, but it sure helped with the sale of the boat
    http://troppo1.mine.nu/photo/photo_one.php?dir=7365616861776b206d73&name=44534346323831352e4a5047
     
    1 person likes this.
  7. waikikin
    Joined: Jan 2006
    Posts: 2,352
    Likes: 124, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    Nice "Mullet", the 80s are coming back, to the tune of http://www.youtube.com/watch?v=X1B_rLIUeWI .... business up front.... party out the back...
     
  8. troppo1
    Joined: Aug 2011
    Posts: 11
    Likes: 2, Points: 0, Legacy Rep: 35
    Location: holland

    troppo1 Junior Member

    Par another question if I may.

    You say above " At 6 knots, you are well below the theoretical hull speed limit, which for you should be very nearly 6.6 knots. At 6 knots you'll be traveling at a S/L ratio of 1.23 a full 9% less then the theoretical hull speed limit of 1.35. This is all without an extension."
    and
    "The additional LWL length will amount to about 1/4 of a knot improvement in the theoretical hull speed"

    Does this mean I would end up with a hull speed of 7 knots and if so would this be still at 2200rpm on the 4-108 burning 3ltr per hr( seems to be the sweet spot on this motor). We just bought the boat and have only delivered it home and pulled it straight out of the water to redo the bottom, build in the bow thruster and the swim platform, so not sure really on the performance yet.

    Not being so tecnically minded, I am used to a multi and scooting along at 15kn under kite, but now I live in Holland and am 10hrs from the sea or lakes, so if I could get 7 knots and not have to burn a lot more very very expensive diesel( you lot have it so cheap over there) then I could get further on the weekends in the same time as I do now at 5.5/6knots.
    is this right or not?
     
  9. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 247, Points: 73, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    To reinforce PAR's advice about not making a transom-extension like this: http://troppo1.mine.nu/photo/photo_...name=5a77656d706c6174206372797374616c2e6a7067
    please bear in mind that wet (submerged) transoms, tend to create bigger stern waves at higher displacement speeds. Deeper the transom, bigger the wave. Big stern waves create additional drag (more fuel consumption, more pollution) and can be a problem if your intention is to travel in your country's canals, where you'll be asked to not create wash. Wash is a problem for both canal infrastructures (create erosion of natural and man-made structures) and for the wildlife (disturbs animals whose habitat is the canal/river shore), so it's always a good practice to try to minimize the wash created by your boat. With a deep-transom boat you'll have to go pretty slowly (4-5 kts) in order to not create wash, while a transom made like PAR has suggested will allow you to go much faster while leaving a more gentle wash.
    Cheers
     
  10. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 116, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    Perhaps you might be better to ask the owners of the boats you sent photos of. What was the performance before and after the extension was added? what was the results for the extensions that had a down angle relative to the ones that just extend the hull lines? At 6 knots or so, the bow has probably not started to lift so the down force added by the stern "hook" will not have much effect on hull trim. Looking at the lift that will be provided by the extension at that speed, it is not a lot compared to the other forces that such a boat experiences. If you are searching for more speed and/or better fuel economy, the extension will probably provide a little bit of both but you may not be able to measure it. I doubt that the wake will be greater but that is complicated by the rest of the hull characteristics and is not an easy question to answer in theory. I have done similar things to boats and it has worked somewhat but only in a narrow speed range like you are looking for, but not in a boat like yours (big and heavy). I am reluctant to dismiss the idea completely but also doubt that is is worth the effort and expense except for utility value as a platform.
     
  11. troppo1
    Joined: Aug 2011
    Posts: 11
    Likes: 2, Points: 0, Legacy Rep: 35
    Location: holland

    troppo1 Junior Member

    tom, the foto's I found on the local 2nd hand sites and the owners had already bought the boat with the swim platform built on and had no idea about before and after( probably bought the boat for the swim platform, very handy here anchoring for and aft in the river, in the locks and most marinas have no fingers so reversing in with 2 ropes tied off at the bow to 2 poles is normal and stepping off the stern to tie up makes life easy, also the reason the the rudder has been enlarged in the first foto and I will also be adding 10cm top, bottom and aft edge, the rudder is useless in the marina going astern ,need 3kn min before it works, boat was built in 78, probably didnt have marina's back then.

    I took Par's advice and used wood under the hull to see what the lines would be and it turns out I have 6cm still in the water at 70cm long. see result here
    http://troppo1.mine.nu/photo/photo_one.php?dir=68756c6c&name=44534346393535322e4a5047

    getting the aft end right might be a bit of a puzzel because the underside of the boat is not flat, all different curves and angles, so I will probably just transfer the transom line to the back and measure down from the top to get it fairly even and looking good.
    foto further down the page is the first hole cut for the bow thruster tube, was easier to do than I thought. Working on both ends at the same time.

    Daiquiri, your right about the bow waves in the canals and rivers, we get rich ******* in huge power boats tearing up the river leaving a 2 mtr high wave crashing into boats and kids playing on the small beaches, I always slow down even at 6 knots when I pass a boat tied up or at anchor
     
  12. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 116, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    If you are willing to do the work and other expenses aren't to onerous, I expect that you will like having the platform back there. Just don't expect any great performance improvement for it. I don't think it can do any harm to either performance or wave making and any change in performance should be positive, if slight..
     
  13. troppo1
    Joined: Aug 2011
    Posts: 11
    Likes: 2, Points: 0, Legacy Rep: 35
    Location: holland

    troppo1 Junior Member

    the shed is heated, the steel is cheap, the work I do myself and the misses will love me for it.
    As long as it doesnt decrease performance then it will be worth it, plus it adds 1000's to the resale value and helps my back with the dinghy and hauling anchors.
    glad I posted here before I went ahead and ordered the steel.
    I'll post some picks along the way.
     
  14. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    With just a couple of inches in the water at rest, your wake will be improved to a degree, though I suspect your bow and mid waves will be fairly substantial, negating much of the improvements in your stern wave.

    You could make the bottom of the extension sweep up to the LWL, so that at rest it's just kissing the water or possably just above it. This would be the best you could get, from a stern that broad and deep. The wake would be as small as you could make it, without major bottom renovations.

    The easy way to visualize these curved lines would be to use a moderately light batten, wedged to the bottom plates and paralleling the boat's centerline (again). Let these run well past the 70 cm mark and bend them up to above the LWL. This would let the water flow "reassemble", in about as orderly and unmolested fashion as you can ask, which is what you want. The additional LWL length will help top speed, but you've added some drag with the thruster and the extension, so it's probably just a trade off. If well thought out, the curved plates could be conically developed, so they'd just weld right on, likely in two pieces (port and starboard of the centerline), plus support structure of course.

    A huge selling point isn't so much the platform, but how well executed the platform is. Does it work with the yacht's styling, does it help the efficiency, flatten the wake, look good, etc. You can "sell" the efficiency and lower wake part, with a ride for a prospective buyer, touting the "before" and "after" preformance figures underway. The visual impact is an area you can make or break the project's usefulness. If it looks like it belonged there all along, then you've done it right. If it looks like it's been tacked on, then it will reduce prospective buyers interest.
     

  15. troppo1
    Joined: Aug 2011
    Posts: 11
    Likes: 2, Points: 0, Legacy Rep: 35
    Location: holland

    troppo1 Junior Member

    I expect to make the bottom from 2 plates and weld them up the middle and top and bottom on the hull,( 2 frames, middle of each plate) then the 2 sides so that I can make the back end from masonite and transfer it to another plate to cut out, same with the top. The top will also sweep off a bit to the sides.
    The bow thruster will have a fairing around the front edges so no disruption to the water flow and no cavitation in the tunnel.
    The normal length of a platform here is 2ft, give or take a few inches, so I expect that part to be ok, and it looks good from on the boat and it is comparable to all the other boats.
    I think for resale purposes I will leave it as it is, people are used to seeing motorboats with a bit of bum still in the water so I need to keep it the same as other boats and also it will be stronger to withstand a hit from behind, the performance impact would be minimal to extend it further up to the waterline, right?
    A question about why they extend the bottom hull line further without building it in, like in this pic
    http://troppo1.mine.nu/photo/photo_...=57696e7465727374616c6c696e67253230322e6a7067

    Does it do anything to help the boat at all?
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.