Making outboard dory 18 10% larger

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Fuzz
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Re: Making outboard dory 18 10% larger

Post by Fuzz »

My OD18 is heavy. In normal fishing trim it weighs around 1750. I often have around #750 of people in the boat. Neither I nor my fishing buddies fit into the little people class :roll: I have a 50hp four=stroke on the dory and it runs in the mid 20's. A OD18 plus 10% with a 60 should run 13-15 pretty easy but TomW will be able to tell you for sure. Looking forward to seeing the build :wink:

fireturk41
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Re: Making outboard dory 18 10% larger

Post by fireturk41 »

I’m calculating a rough 2100lbs including my motor. I’m looking to launch in Beaufort NC and do mostly island hopping, although occasionally stay inshore in the ocean once I determine my range on the boat if the weather allows. That’s why I want it to be able to handle some chop. I have no qualms with it cruising faster but when you start hitting waves speed starts beating me up. Right now I’ve got an 06 evinrude 60hp etec motor in storage for it.

I guess I’m also asking how these handle in rougher water with the length and beam they come with standard in the plans. I know most people run flat bottom skiffs here with 19-21 length and 8ft beams

Fuzz
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Re: Making outboard dory 18 10% larger

Post by Fuzz »

How they handle in rough water depends a lot on the skipper. Cracker Larry had his out in weather that was nuts! Nuts to the point where much larger boats were going down. CapeMan takes his for some pretty long offshore trips and it seems to do ok. Lots of folks will talk about them being flat bottom but I have not found that to be a problem. Just about all boats in the 18-20 foot range will have to slow down about the same time.

fireturk41
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Re: Making outboard dory 18 10% larger

Post by fireturk41 »

Do you think the 10% in length and beam will make any notable difference? It honestly is much easier to build to the plans that already exist

Fuzz
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Re: Making outboard dory 18 10% larger

Post by Fuzz »

I think it will make a bunch of difference. Scaling up 10% yields a 30% larger boat. But I think the hull is well suited for a larger build. I know there are many different thoughts on hull side height but if I had it to do over I would have made mine 6 inches or so higher. But my boating is in cold water so spray is no fun and falling over the side can very quickly lead to death.

TomW1
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Re: Making outboard dory 18 10% larger

Post by TomW1 »

fireturk41 I will run your boat at 2100lbs with an Evinrude 60HP motor on it. Give me a few hours. Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

TomW1
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Re: Making outboard dory 18 10% larger

Post by TomW1 »

Okay I got it done faster than I thought as I was able to find the Evinrude specs easier than I thought. Her they are if you ever need them: file:///C:/Users/Tom/Downloads/ETEC-60-HP-Outboard%20(3).pdf

Now for the boat calculations: 2100lbs, 60HP Evinrude, OD18+10%, 19.8' long, 7.7' wide. Top speed 34-38 depending on load, 36.6 for 2100. best cruise around 24mph but you will have to use your rpm and mpg guage to determine this. Some boats get there best fuel mileage at 3500rpm's others at 4500rpm's. It all depends on hull shape and motor type and gear ratio. My calculator through out 3 blade props for your weight of 14.5x18 or 14x19 and for a 4 blade prop of 14x16. This makes sense as a four blade prop always requiresi a drop in pitch for the same performance.

One last thought make sure you stop the skeg 2' from the end of the transom so that you have clear water flowing to the prop, not disturbed water that you would have if the skeg ended at the transom, yea all the pictures show the skeg ending at the transom but that is an error by the drafter.

Hope that helps you. Please ask if you have any questions.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

TomTom
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Re: Making outboard dory 18 10% larger

Post by TomTom »

fireturk41 wrote: Mon Jun 06, 2022 1:04 pm I’m calculating a rough 2100lbs including my motor. I’m looking to launch in Beaufort NC and do mostly island hopping, although occasionally stay inshore in the ocean once I determine my range on the boat if the weather allows. That’s why I want it to be able to handle some chop. I have no qualms with it cruising faster but when you start hitting waves speed starts beating me up. Right now I’ve got an 06 evinrude 60hp etec motor in storage for it.

I guess I’m also asking how these handle in rougher water with the length and beam they come with standard in the plans. I know most people run flat bottom skiffs here with 19-21 length and 8ft beams
Have you considered the FS19 or one of the Pangas say the 22 or 25? Relatively simple build, no need to scale the plans, similar power requirements and will handle chop better.

I have an OD 18 and a C19 that I stretched - I definitely notice the chop/ flat bottom in the OD - so you basically have to drive it a lot more appropriately to the conditions than one with some V.

Also - the minute you start deviating from the plans, the build time goes up hugely. What you might gain with a bigger OD, you will lose in time spent and extra materials that could be used on another design.

Others may feel different but there is is for what it’s worth.

TomW1
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Re: Making outboard dory 18 10% larger

Post by TomW1 »

Another boat you might consider for your motor is the FS19. With the 60HP at 2100lbs it would have a top speed in the 35mph range with your motor and with the v-hull ride better. This is one of Jacques newer designs, but those who have built it like it. Beaufort looks like a fishy place with all the islands and the slot to go offshore.

Either boat will do you fine Just depends on the ride you want when the wind kicks up. At 2100lbs you are going to lose 1 - 2" of draft on the OD so the draft of the 2 will be pretty close.

Good luck on your decision, Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

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