Hull modifications

Discussion in 'Projects & Proposals' started by MKP, Jul 3, 2008.

  1. MKP
    Joined: Jul 2008
    Posts: 35
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    Location: Benicia, CA

    MKP Junior Member

    Hello Guys,

    First post here. I've been looking for some input with a project i am beginning. -Hoping you all have some insight for me.

    I have a 32' Roberts ex-bristol bay gill netter. I have converted the interior and top side to a plush sport fishing boat. The vessel has twin 250hp Cummins 6BTAs and is about 20k lbs. WOT is 14.7kts at 2800 rpm with a light load. Wheels are 24 x 18. Shafts are 1.75". She is squirrelly going downhill.

    My current priority is to improve economy and speed. Originally the vessel was equipped with skeg keels (one on each shaft) huge 8" x 1/2" steel shoes and 1/2" steel cages around the rudders. You can guess where I am heading with this. I want to reduce drag on the bottom of the boat.

    My plan:

    Remove both skeg keels
    Remove shoes
    install longer shafts
    remove rudders
    install new smaller rudders further aft
    increase wheel pitch to fit
    add trim tabs - one on each side and one on the centerline
    swap wheels for outward turning rotation

    I am curious what ideas, experience you guys have offer. Am I moving in the right direction here? Is the work going to be worth the gain? Ideas on trim tab placement?

    Thanks in advance,


    Attached Files:

  2. lazeyjack

    lazeyjack Guest

    First the POS
    She has potential to cope with chop, nice fine entry
    But my God!!
    All that stuff at the stern , and on the bottom, is well agricultural to say the least
    yes IMO,you a re on the right track, get rid of it all
    You are steel so you have advantages
    Once all those early 60, Chevvy finns have gone:))
    Put p struts to support the shaft, double up inside with plating to reinforce this area
    You have big power and big screws, what is gear ratio
    Then can you post rudder sections
    This is one neat project, (for some of us) as we don't actually have to do, it, but can sit back and watch you labour away
    Now you will have less drag, you will have aboat that does not try steer itself etc, plus you will shed abt 600 lbs useless steel
    cheers Stu
  3. MKP
    Joined: Jul 2008
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    Location: Benicia, CA

    MKP Junior Member

    Thanks for the reply Stu.

    Yes, the boat was a bit overdue for a cleaning. It sat for almost two years during the topside retrofit.

    The boat is glass -very heavy glass. Not steel. Some places the glass is near two inches think. The shoes are all made of steel.

    Gear Ratio is 2:1

    What is a P Strut? I was planning on V struts for extra strength and due to interior access points.

    How close should the rudder be to the wheel? Is there a basic rule of thumb here?

    What about rudder size? Old rudders were steel plate 22.5"h x 20"d with 1.25" shafts. I'm thinking about a free hanging spade rudder with 1.75" shaft and app 20h x 13.5"d.
  4. Guest625101138

    Guest625101138 Previous Member

    I can see Stu salivating at the prospect of watching this project develop. I also think it would be entertaining and educational.

    I have not done a modification that goes even close to this but I will provide some input for your evaluation.

    My estimation of the drag components are as follows:
    Wave drag 70%
    Transom drag 20%
    Frictional drag (clean bum) 10%

    I believe the form is one of semi-displacement that will not plane easily. The line of the hull does not promote lift in the mid section where it is needed. Would be interested in your observations here?

    By cleaning up underneath there will be a small proportion of the wetted surface removed so a slight reduction in frictional drag. The reduction in weight will likely have a greater beneficial influence by reducing wave drag but the weight reduction is not significant in proportion to the overall weight.

    Removing the aft keels may make it more skittish down hill as the stern will slide more readily. May not roll quite as bad because grip is reduced. Personally I think the deep entry is the big contributor to poor control down a wave.

    If motor weight is shifted aft it may help handling by lifting the bow but the transom will be deeper and hence increase drag there.

    If the underwater is cleaned up with all excess weight removed you might find 0.5kt. Probably about the same as removing the sea life from the hull.

    I could be wrong - I have been wrong before. Certainly worth another opinion from someone who has tried something similar. For me I don't think the likely carrot warrants the effort.

    Economic speed is likely to around 8kts but the installed power is huge for this speed. Makes me wonder if it was ever repowered.

    Rick W.
  5. TollyWally
    Joined: Mar 2005
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    Location: Fox Island

    TollyWally Senior Member

    The Roberts hull is a fine boat, well respected. It sounds like you are willing to put quite a bit of effort into this project. A few things to note. Little fishboats like this are designed to be able to take extremely harsh weather conditions and haul weight back to port at adequate speed. They head out filled with ice and fuel, bow down and come back with ice and fish dragging thier asses. The engines are a bit forward to make room for the all important fish hold and to help lever the *** end up under a big load. She probably feels squirrely to you because she rides bow down light.

    The skegs and shoes are to help keep the net out of the wheel and let her dry out on the flats. The tunnels allow a bigger more powerful wheel for hard pulling and pushing a full heavy hold full of salmon home that still is protected by the skegs. Those tunnels probably don't help a bit to get her planing.

    Others may correct me but I think trim tabs will be more effective in pushing the bow down rather than prying the stern up. As long as you are willing to spend the money and effort a few things come to mind. Shifting the motors back for improved balance, trimming but not removing the skegs, and filling in the tunnels. Possibly moving the tanks. She goes out with full tanks and comes home with lighter ones. The weight of the fuel burned is to partially help balance the boat when the hold fills with fish. Maybe with the weight shifted back towards the stern trim tabs will be effective for tuning the trim angles.

    You have a fine boat that is a little bit rough around the edges but well built. The mechanical things that matter will be heavy duty, first rate, hell for stout, and better than yacht grade. Your hydraulic system will make all kinds of things possible and powerful. Boats like yours routinely operate in weather conditions that would absolutely terrorize the vast majority of boat operators in the world.
    She is never going to be a speed demon, 15 knots in present trim is respectable. I bet you can do better than 20 optimized. You can do 10 or 12 in rougher conditions than you can imagine and 6 in gale conditions. I wish she was mine! :)
  6. MKP
    Joined: Jul 2008
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    Location: Benicia, CA

    MKP Junior Member

    Wow this is some great info. Exactly the input I've been searching for. I have thought about moving the engines aft a few feet. Maybe spinning them around to drive a V-drive.

    You are correct, the boat is bow heavy. Even with a load of 400 gallons of fuel and 150 water the stern is a bit high. All tanks are as far aft as possible without infringing on the steering gear. Ideally, where should the center of gravity to be? Aft of midships?

    Mr. TollyWally you seem to know Bristol bay boats well. Have you fished up there? Do you know any of Roberts' history. Who were they and where did they go? It is difficult to find info on these boats.

    What do you mean by trimming the skeg keels? Maybe trim them to below the shaft log?

    Rick, Can you explain to me the difference between wave drag and transom drag? Transom drag is the low pressure caused when the water breaks free of the hull? Wave drag is breaking free from displacement mode to planing?

    Thanks again for all your guy's insight.

  7. Guest625101138

    Guest625101138 Previous Member

    All moving hulls experience wave drag. For a hull with low length to beam it is the dominant drag factor between displacement speed and planing speed. So your description is fine.

    Transom drag only occurs if the hull has an abrupt cut-off. This creates a low pressure zone behind the stern that contributes to drag. A canoe type stern has close to the same pressure as the bow of the boat as there is no abrupt change. However the canoe stern is wasted on a hull intended to plane.

    If you move weight aft to sit the stern down it will add transom drag at lower speed. Interestingly it also will require more power to plane if weight is moved aft.

    I agree with TollyWally. It is a fine boat for its intended use.

    On the numbers you have provided and pictures it has a lot more installed power than that required for the speed it is achieving if were the ideal shape for full planing. This also supports my view that the hull is really not meant for speeds up around 20kts. It is hard to see the exact shape of the hull where it transitions from the bow to the tunnels but it does not look like a good planing surface.

    Rick W.
  8. MKP
    Joined: Jul 2008
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    Location: Benicia, CA

    MKP Junior Member

    Shoes are gone. Keels are 1/2 off. Cut to just below the shaft logs. Haven't decided to cut them off completely yet.

    Rick here are some more pictures of the hull. Will this thing plane?

    Attached Files:

  9. Guest625101138

    Guest625101138 Previous Member

    It has enough planing surface to plane if the power is up to it.

    The things that will not help it plane are the deep forefoot and the prop tunnels. I have not tried to analyse the impact they have but they must be significant at is has ample power to do 20kts if it was a flat run aft with shallow entry. Alternatively you have the weight wrong or the props are underpitched to utilise the power. (The latter could be the case if it is designed for heavy load in heavy weather) I calculate you would need 20" pitch to get 20 kts from 1400rpm at the prop.

    It is sometimes sobering to do a weight check if possible. Does the vessel transporter have a load cell? Did they check weight when lifting?

    Do the engines rev easily. You said 2800rpm. Is this rated rpm. Do the engines labour in a head wind or she will hold around the 14kts in any condition.

    The big props are probably the best set up I have seen short of a tug for the sort of drag required to plane. This of course is a mixed blessing as they have very high efficiency due to the diameter but the tunnel detracts from lift for planing.

    I have been in smaller planing hulls with an entry not quite as deep as yours and they are scary coming down a wave. You get caught between backing off to ease into the trough and lose steering or keep powering into the back of the wave and risk broaching.

    Rick W.
  10. MKP
    Joined: Jul 2008
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    Location: Benicia, CA

    MKP Junior Member

    The travel lifts scale was not working at the time I hauled out.

    The engines are rated for 2600. the wheels are under pitched for pushing heavy loads. The engines never labor in any condition. I plan to increase pitch in increments. My prop guy is currently putting a heavy cup on the wheels and I have some 24 x 21 wheels that came with the boat. I want to leave about 100 - 150 rpm spare.

    I don't expect this boat to ever do 20 knots. I would like to cruise at a decent 14knots though. I currently cruise 9-10 because it takes twice the fuel to get the extra 4 knots. The boat sort of planes at 14.5 knots but at WOT. I would kill to have the boat cruise on a plane at 14knots at 2100-2200rpm.

    Odd as it may seem surfing down larger swells the boat does pretty good. Its the smaller wind chop and steep swells that make for tuff steering. At times it too much for the autopilot.
  11. Guest625101138

    Guest625101138 Previous Member

    Some drive combinations are actually over pitched to get the engines into the most efficient range at cruise. Trouble with this is that they labour in heavy weather or if pushed hard.

    I wonder why it came with spare 21" pitch. These seem more like I would expect for the hull and the installed power.

    Altering the pitch can have a small impact on fuel burn but it is hard to escape from the physics that it takes more power to go faster. There can certainly be a point where the drag actually drops away as the boat gets on the plane and you get almost constant power requirement. This is probably around the 14kt mark. The point where the hull levels out.

    The viscous drag does become more significant once on the plane so getting rid of the extra area will have increasing benefit as you try to go faster. The numbers certainly suggest you could get to 20kts with the installed power and heavier pitched props.

    The tube bundles under the hull probably don't reduce drag either. Fairing block in front and behind them would help providing it does not impair their function.

    Rick W.
  12. Guest625101138

    Guest625101138 Previous Member

    The attached is the Savitsky model approximating your hull. The model that this is based on has cleaner lines though so would likely underestimate. The power shown is for the hull and does not consider prop efficiency and other appendage drag or windage. Typically you could expect about twice the power level shown but you have almost ideal prop diameter so it may not be that bad.

    It does show a slight tapering in the rate of increase in drag (thrust) at the start of the curve and levels a bit more over 20kts.

    Here is the link if you want to play:

    Rick W.

    Attached Files:

  13. lazeyjack

    lazeyjack Guest

    yes you are so lucky Rick has picked up on this one,as I knew he would
    I lay thinking abt this in bed, and suddenly those flared bows came to vision, and I thought , WHOOPS not steel, I was so engrossed in the bottom and as I dont know anything abt sticky stuff, will watch, when you get to the eng I can assist more
  14. MKP
    Joined: Jul 2008
    Posts: 35
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    Location: Benicia, CA

    MKP Junior Member

    Wow Rick that graph is a bit over my head. I'm gonna have to digest it a bit more. Very interesting link though. I am learning all sorts of new things about hulls. I ordered The Nature of Boats and the propeller handbook for some added insight. This is really interesting stuff.

    I am not sure why the boat originally had 21" wheels. I think it was over propped, but a few sea trails will tell. The previous owner was good friends with a well know diesel guy in Oxnard, CA. I believe they installed the new wheels to get an extra 200 rpm for engine longevity.

    The Keel coolers will hopefully be removed in the next few months when the vessel is repowered. I have applied for a grant from the state to install cleaner burning engines. We are planning for QSB 305s which have a bit more hp and a lot more lower end torque which will help my cause.

    So my question now is should I trim off the rest of the shaft keels? Or maybe fair in what I have just below the shaft log? You can see the keels are about 8" in diameter and block significant water flow across the wheels. I'm thinking filling in the tunnels will be a tremendous amount of work. The change in shaft angle would have to be so extravagant that it may be counter productive. The angle of the shaft relates heavily to the propellers efficiency correct?

    Thanks for the link Rick. I assume that calculator will consume most of my evening.

  15. Guest625101138

    Guest625101138 Previous Member

    You really do not want to change the prop set up. To have shafts that are near horizontal swinging nice big props is a true blessing. With well selected props you could expect prop efficiency in the high eighties. This is very unusual.

    Anything you can do to fair out the appendages can give significant gains relative to unfaired bits. However remember the big drag components come from the stern and general shape at semi-planing speed. Improving the underbody mostly helps the top end when it is on the plane.

    Props are now exposed so prone to damage going astern.

    The rudder shaft will also need to be able to take bending now. Also I think the "V" strut limited the range of the rudder. This is handy to avoid overloading the steering when going astern. If I am correct here you will need some form of steering stop. Rudder get big forces going astern at any speed.

    I would be wary about exposing the shaft any more.

    You certainly got stuck into it fast. As Stu said it will be interesting to follow the progress.

    Rick W.
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