Hull Wedge / Hook ?

[Frederick]

Hello,

I am new to this forum and new to boat building. I am currently designing a boat that will be a variation of a modified Dory as it has a flat bottom with flaired sides. The boat will be 18 feet long and hopefully weigh under 300 lbs. That is the goal. The use will be for lakes and for fishing shallow inlets. the rear transom will be traditional dory style but much wider to accomodate a motorwell and motorwell outboard transom located 24 inches forward of the rear transom. Outboard power will be a merc 9.9 hp. Not going to go fast, but my favorite lakes have hp restrictions. The design is complete except for one detail. I am considering building a hook or wedge effect into the aft hull section between the motorwell transom and the rear transom to provide lift and stability so the boat doesn't porpoise. I know that T.N. Simmons designed and built sea skiffs with this aspect, but those are a slightly different boat. I also know I could just place trim plates on the rear transom, but I am attempting simplicity and wanting to avoid extra hardware.

I have researched for design aspects of how much of a hook or wedge to use for a typical ? foot boat, but I can't seem to find any information. To run the hydrostatic analysis is beyond my capability both skill wise and personal computer capacity.

Does anyone have any experience with this, or does a table of charts exist that can assist in determining the amount of hook/wedge needed.

Thanks for any feedback. Frederick

[tom28571]

I doubt you will find factual information on exactly how much hook or wedge is needed. The easy route is to build the boat and then see what is needed. You can add wedges on each side beginning with small ones until you are satisfied. I have just cut some wooden wedges and taped them in place with thin plastic tape for experimentation. A little goes a long way so I would not want permanent ones in place that had to be ground off. After you find what is correct, you can mold on some permanent ones.

[Willallison]

With 10hp, 300lb of boat, another 200+ of gear and crew, is this boat likely to get on the plane?
If not, then wedges are probably not necessary. Also, if the boat has a completely flat run aft, then unless all the weight is concentrated near the stern then wedges, or hook, shouldn't be needed.
As Tom suggests, build the boat 1st. If you find that it needs some sort of trim adjustment you can always add it later

[Frederick]

Thanks for the advice Tom & Willallison,

I have given that some serious thought before and the only part I don't like about it is that the wedge would stick below the garboard, not much, but you would still see it. I know, that part is under water most of the time, don't be a nitpick, but that is me, everything in its place. So, my thought was that if I added them later I could taper/feather the outer edges so it wouldn't just be a piece of end grain sticking down. Maybe a convex wedge would have some value in creating a channel that would assist with tracking and possibly stearing. That is why this is an art as well as a science I suppose.

As far as weight goes my target is 275 lbs. The motor is about 60 lbs. Me at 210 and another passenger and some stuff should top about 550 to 600. Based on projections it should plane but that is figuring in a hooked/wedged aft section to provide starting lift. Not looking for a fast plane, just a plane and no bow up porpoising syndrom. Level glide so to speak.

[PAR]

Unless you modify the bottom, to remove the up turn in the aft rocker or are using a power dory shape (there are many dory shapes) it's unlikely you'll be planning with your estimated weight and typical dory narrow length/beam relationships at that HP. The bad habits you have spoken about, with regard to the need for a "hook" to help correct a yet developed issue, seems premature. Most of these bad manors can be corrected with trim. If after launch, trials and trim adjustments, she still likes to stuff her nose repeatedly, then you can consider wedges, adjustable tabs, fixed tabs or wedges, etc. A sharpie would be better suited to low power planning, then a dory form.

[Frederick]

Hi PAR,

Yes, when I said modified the rocker has been taken out completely in the aft section, and the bow has an upsweep starting from the third frame which is the first 3+ feet. So basically 80% of the hull is flat as a pancake very similar to a power dory. So this gives adequate planing surface if my calculations are correct. My thoughts on wedging or hooking the aft portion of the hull, or kicking it down so to speak is to provide a little extra uplift from the hole and to accomodate days when it will just be me at the stern stearing the boat. Even a completely flat boat will want to squat a little with one man in the stern. Had a jon boat years ago, 12 footer, just me and a 5 hp Johnson, still had the nose to far in the air for my tastes. Based on fluid dynamics and pressure created by the kickdown similar to an airplane wing it should settle the bow down. I know that too much trim can cause sensitive steering to be an issue, but with a flat bottom boat and a small keel strip sensitive steering has never been an issue. Generally, you have to plan way ahead for your turns so making it a little more sensitive might be a good thing. I just don't want to create unecessary drag with to much hook/kickdown. I was hoping there was a rule of thumb to applying trim either through mechanical tabs or the amount of hook/kickdown in the aft section.

Anybody know about a rule of thumb like that.

[PAR]

I still think the hook is a premature, unless you have some experience with this hull shape and similar models have shown a tendency towards this problem. If this was the case, then I would change the hull shape to gain additional bearing aft, rather then burden an under powered hull with the drag of a hook. 2 degrees of bow angle up would be an ideal target, but imposable to calculate with precision. There are some rules of thumb, but these generally fall under more conventional hull shapes and are very broad. The flat bottom form is a very efficient planning shape, though with limited power, weight, entry shape/angle and trim would be my primary concerns during design development.

Posting a limited set of lines would help a lot in understanding your design ideas.

[Willallison]

Quite apart from the fact that, if designed properly, you shouldn't need wedges, and given that the variable weights - ie crew and gear - are going to make up such a significant proportion of the overall weight, I'd be looking at weight placement. And given that these weights are going to vary so much, I'd be adding adjustable tabs later (if they are needed at all).
For some basic info on the size of tabs (which could be used as a guide for wedges too, I guess) take a looke at the bennett tabs website - they have a fair bit of general info there...

[messabout]

Consensus best advice: get out a new sheet of paper and draw a sharpie as Parr suggests. Leave the bottom flat, no hooks or other gimmicks. The 9.9 will have plenty of power to drive the boat into the 20 MPH region as long as the total weight does not go too high. My 9.9 Johnson produces 16.3 HP at the prop shaft according to a dynomometer run. Your Merc will probably do as well if has a displacement of 250cc or more. Thats plenty of power to do what you want if the boat is thoughtfully done.

Read Willallison and Parr again. Weight distribution will take care of the porpoising problem if it exists at all. If you are committed to the dory design then you must accept that it is not the best performance planforn. Here it depends on what you call a dory. My minds eye sees a Banks or Chamberlain dory. Lovely to look at, yes. High performance with low power, no. If you are thinking of something like a Bartender it will probably need more power than you are prepared to provide.

A flat bottomed boat will most surely pound at speed, particularly if you have the forefoot up. If you put the forefoot down the boat will be twitchy when quartering even small waves. That can be a dangerous excercise. Accept the pounding and keep the nose out of the water. This is not to suggest that you abandon the flat bottom.

You can not really compare your experience with a 12 foot Jonboat (mortar box) to an eighteen footer. The longer boat will run flatter by virtue of its own length and inherent weight distribution. One more comment about porpoising...Don't make the boat too wide.

[Frederick]

Thanks for the feedback messabout,

I have decided to get off the hook and just keep it simple. If I had access to the type of hydrodynamic science needed it would be interesting to be able to design in a hook for plane control, but since I don't, I am going to stay with a flat bottom. I agree on the width issue, but some disagree. The boat will be 40 inches instead of the typical 48. Some have said that will make it tippy, or tender. I disagree. I think it will smooth out the plane and with a modest keel strip let it turn better. The reference to it being a dory style boat is the it will have flaired sides and a set back transom with an internal motorwell transom.

Like you, I think the 9.9 hp merc will be just enough power. I should finish my third model this weekend. I will post pictures when done.

Thanks again.

[LP]

Just a little something to look at by a well known designer.

http://www.dngoodchild.com/divide_for_power_boats.htm

Old but could be updated to plywood/stitch-n-glue techniques.