Amateur needs guidance. Experimental Build

[messabout]

OP needs to tell us more about what he calls a dory. Are we talking about a Banks nesting dory, A Swampscott, a St. Pierre, or some other kind of boat like the Northwest planeing boats that are called dories, or something else entirely.

 

[viking north]

That would certainly help along with the basic dimensions. Ya the term dory has been used to describe some weird and wonderful craft.

 

[AVBoater]

Hey all, just got a chance to sit down. So the dimensions are 35" at center and 8" at the stern, with a 12' span. I will include some pictures of the bottom. We have not yet set ourselves to 1 gear box or engine so I don't have any pictures of those. We are estimating around 850 pounds with gear and passengers.

 

[kerosene]

that is not a planing hull and 1 hp is closer to what you need.
Being such a small boat I would recommend something ultra simple. Like paddles for reverse - voila no gearbox. Or a trolling motor with a battery and small solar panel on dock to charge it (depending on use).

Tell us a bit more broadly what is the goal? coast for 2h while casting lures? going to places? 2 people on board is all I know so far.

But yeah engine you described is unnecessarily big - it will not make the boat go any faster than a small one.
http://en.wikipedia.org/wiki/Hull_speed

 

[DCockey]

Hey all, just got a chance to sit down. So the dimensions are 35" at center and 8" at the stern, with a 12' span. I will include some pictures of the bottom. We have not yet set ourselves to 1 gear box or engine so I don't have any pictures of those. We are estimating around 850 pounds with gear and passengers.

Looks like a traditional dory though wier than a banks dory. I'm assuming by "a 12' span" you mean the bottom is 12' long. That would put the overall length around 15' to 16'.

I'm guessing that the boat hasn't been in he water since it is unpainted and inside. Small boats suddenly shrink and are not nearly as large when they are taken outside and put in the water as when they were inside.

A 2 HP engine would have more than sufficient power to drive it as fast as it will comfortably go. A very small gas outboard or electrc trolling water installed in a well trough the bottom will probably be much simplier to install than an inboard which will require a shaft, shaft log to go through the bottom, propeller and a rudder. Propellers for small engines are not easy to find and you could easily spend more on installing an inboard then the total cost of a 2HP gas outboard or electric trolling motor. On the other hand some folks prefer the asthetics of an inboard.

 

[viking north]

Traditional dory shape she is. In the early days when gas engines became available many were converted into what we called "Motor Dories" However alot of interior space was taken up with the motor house /fueltanks/drive systems and as a result new builds of the motor dory resulted in a much larger craft up to 24ft. Later when outboards became available many regular size dories 14 to 16 ft. bottoms (16 to 18ft. LOA) were converted to outboard power by installing a motorwell . Some where i have a set op detail plans on the work involved which is well within the novice's ability. I will attenpt to search them down and post them tonight. If it's only engine power you're interested in this might be your best route. However if it's the old school appeal you desire have a look at the Reo set up for guidance.
P.S. The traditional length of a dory was always expressed by it's bottom length, why I don't know other than this was the first part built.

 

[Dave T]

I assume the reason everybody is recommending such low horsepower is because of the small bottom area and if it was brought up on plane it would be unstable and be dangerous. A small outboard or electric trolling motor would probably be best as an inboard setup would take up a lot of room and a small outboard would probably be cheaper in the long run.

For the experts
Now that we've seen the pictures and know the dimensions what is your opinion of the what maximum safe top speed for this boat would be ?

Dave T

 

[AVBoater]

So Ive been doing a bit of reading about planing and types of planing hulls. To help me better understand i have a few question; Could the hull theoretically plane at a certain point (defer from thinking about force taken to get it to that speed)? And if so then the prop diameter I'm limited to wont allow me to? If not, is it that i do not have enough surface area to plane on? Do i need deadrise to create vertical force to get the front of the hull out?

 

[daiquiri]

Planing ability depends heavily on the shape of the bottom, on the vessel's weight, on the trim (angle of the bottom respect to water surface), and on speed.
In theory, most hull shapes could plane if they were sufficiently light and driven at a sufficiently high speeds. The problem is - if the hull shape is not suitable for planing, making it run at high speeds would require very high engine power, and high engine power requires a big and heavy engine. A big and heavy engine adds more weight to the boat, which then requires even more power then before. And so on...
If you could tow your empty boat behind a powerful and fast vessel, you would probably see it planing. But if you tried to do it while sitting in the boat, it would be much harder, if not impossible to do. Thats because your boat has a very narrow bottom, with maximum beam in the midship.
So, you might try to increase the trim in order to make it plane, by moving yourself all the way back. But your boat has very little volume and nearly no bottom surface in the aft, so again it would have no means to generate any significant dynamic force... All in all, that hull just isn't suitable for planing, without some modifications. Youre bound to go at displacement speeds, around 4.5 kts.

 

[DCockey]

AVBoater, do you have any experience on the water in a dory like the one in your photos?

 

[AVBoater]

No experience yet. We found the plans we liked and added a bit more beam to it. We have been in the midst of discussing if we wanted to add power and how much based of feasibility. Which brought us here to discuss it

 

[DCockey]

Where did you get the plans?

 

[DCockey]

You will probably find the dory has considerably less intitial stability than similar length motorboats and sailboats.

 

[daiquiri]

Now that we've seen the pictures and know the dimensions what is your opinion of the what maximum safe top speed for this boat would be ?

My opinion is in my first reply: with a 3.5-4 HP motor it should run at some 5 kts (perhaps a tad more - say, 5.5 kts with a big wave left behind), or 4.5 kts when the sea gets choppy. Not that I'd like to be in that boat when it gets choppy, though...

Cheers

 

[Dave T]

Maybe somebody elses thread isn't the right place to do this, being there are a lot of experts looking at this thread I'll put on a few pictures of my boats hull design. The design is completely my own I didn't use any plans only my own ideas of what I thought would work this is the first and only boat I have ever built, I don't claim to be any type of expert at boat design so maybe some expert opinions would help in case I ever built another boat or somebody wanted to copy it. She's 12' long with a 6' beam twin hull, each hull is 32" wide with a 7" tunnel between them the hulls and deck are filled with closed cell foam. I do most of my boating on the Mississippi which sometimes is very rough. Ive had no problems so far, she planes real well handles large waves and is very stable. She's fairly heavy do to the materials used and the amount of equipment I carry 3 large deep cycle batteries, trolling motor, 6 gallon gas tank, 30 horse outboard, 5 gallons extra fuel, anchor and usually a large cooler of beer. She weighs right at 1000 lbs. with equipment and without passengers. Top speed so far is 26 mph with just me aboard according to GPS.
AV that's a great looking little boat you built looking forward to seeing it on the water and seeing how she does keep us posted.

Dave T