Making outboard dory 18 10% larger

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fireturk41
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Making outboard dory 18 10% larger

Post by fireturk41 »

I tried looking at the link for adding 10% to the size of the boats. I ordered the outboard dory plans and want to add 10% mainly for the purpose of having another foot or so and a wider beam. Is it fairly straight forward as adding 10% to everything else or should I be wary that it’ll significantly impact the project

fallguy1000
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Re: Making outboard dory 18 10% larger

Post by fallguy1000 »

Read this and ask specific questions.

https://boatbuildercentral.com/support- ... -plans.pdf
My boat build is here -------->

viewtopic.php?f=12&t=62495

TomW1
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Re: Making outboard dory 18 10% larger

Post by TomW1 »

Yes please read fallguy's reference above, the best way to do it is to both lengthen and widen her. Side heights do not change unless you want to change the look or add a couple inches for safety, but again you are playing with wood amounts. Generally you will need more wood, fiberglass epoxy, and everything else. You can look at your plans and you will see how tightly everything is layed out to avoid wasted wood.

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

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BarraMan
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Re: Making outboard dory 18 10% larger

Post by BarraMan »

The simplest way to do it is to buy the metric plans and multiply all the dimensions by 1.1. Personally I would include the dimensions for the height of the sides to keep the boat in the right proportions - but that is your choice. In my case I added a bit more to the sides because I fish in crocodile country.

Yes, you will use more wood, glass, epoxy and paint - just goes with the territory. Cleverly nesting the parts can be a challenge but the cost of a bit more wood is minor in the greater scheme of building a boat.

Check with Jacques about whether you need to add an extra layer of glass in places, ie the bottom.

A word of warning! Its easy to say that you will upscale by 10% and its quite easy to do - but understand that you will end up with a substantially BIGGER boat! How do I know? 8O My boat is based on the Mangusta 20 hull and went from 5.95M length and 2.15 max. beam to 6.7M length and 2.5M max. beam! This will mean a bigger trailer and possibly a larger hp motor!

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Re: Making outboard dory 18 10% larger

Post by piperdown »

BarraMan wrote: Thu Jun 02, 2022 2:51 am
Check with Jacques about whether you need to add an extra layer of glass in places, ie the bottom.
What BarraMan says about checking with JM is an important consideration!
Eric (aka, piperdown)

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Re: Making outboard dory 18 10% larger

Post by TomW1 »

If he goes over a 70HP he should add another layer of glass on the bottom of the OD. Jadques lists 70HP as the max. I have a 70-2st on mine and cruise at 24-26mph max at 34. A 90 would push into the low 40's increasing the bottom pressures more than one layer of biax can absorb.

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

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Re: Making outboard dory 18 10% larger

Post by jacquesmm »

It can be done but read what Barraman says about it: a boat scaled up by 10% in 3D is 30% larger: 1.1^3.
I would scale all dimensions and do it in metric.
Also, check how scaled panels will fit on standard plywood sheets: you will need much more plywood than 10% extra.
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Re: Making outboard dory 18 10% larger

Post by TomW1 »

Piperdown asked a question of Jacques down in Builders Power Boats section which should have been asked here. Here is his response:

From Jacques,
"When you scale up and down, my 10% limit does not require any changes to the scantlings but, in many cases, I recommend extra glass layers when the builder installas a larger engine I would take it case by case."

The OD18's stretched max speed with Jacques max reccomended 70HP motor is in the low/mid-30's, with a 90HP it is in the mid-40's. At this point since it is a flat bottom boat we need to check with Jacques if a second layer of glass is needed. From my knowledge and having talked with Jacques and seen many boats built on here, my answer is yes if you go over 70HP. This stretched boat can handle up to a 115HP if you wish but that would be excessive imho.

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

fireturk41
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Re: Making outboard dory 18 10% larger

Post by fireturk41 »

I was finally able to get the link from the second post to load. Darn computers. I called to ask and they said that 10% should be fine, but I will probably add a second layer of glass to be safe. I plan on using a 60hp evinrude and hope to achieve a cruise speed of about 13-15kts. I wanted a little more length and beam because it does tend to get choppy here in the afternoons

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Re: Making outboard dory 18 10% larger

Post by TomW1 »

Which lakes do you go out on B. Everett Jordon or Falls Lake, they both look fishy. I have returned in class 2/3 water in my OD18 without any problem on Lake Fontana which looks more like Falls Lake which runs more east/west than Lake Falls. Yes I had to slow down.

A cruise speed of 13-15 will put you right about the hump for this boat the most fuel ineffiecent area of the motor. Your basically trying to push the boat through the water to get on plane. Yamaha has a boat test for a 17'8" boat but only 6'1" width wide where the most effeicent cruise is 3500 rpm's at 20mph for max gas range. This 3500 to 4500 rpm range is normal for max gas range in all the boats I have come across and anything below 15mph is a gas killer except for displacement boats due to there very different hull shapes

If you want some rough calculatios on what your boat I would be glad to help. Give me a list of the following:

OD18 x 1.1 I will calculate weight
Which motor you will use, I have all the specs: Just for your info lead times on motors are 6-7 months at this point in time Look real hare at the Suzuki over the Yamaha as it has several advantages.
Gallons of gas you will put in her x 6lbs typs of tank and weight
Hardware weight on boat, anchors, cleats, electronics, etc I will add a constant I use for gas lines, PVC piping, cable control lines and things of that nature.
People weight
Cooler weight and ice
Fishing gear
People toys like skis, tubes
Water, beer, things to eat

We want to get as close as we can to the weight of the boat as it leaves the dock as we can.

Right now just do estimates we will do this again when we size your propellor. Take your time I am still on vacation and don't even have my caculator. :lol:

Hope this helps, Tom
Last edited by TomW1 on Mon Jun 06, 2022 1:20 am, edited 2 times in total.
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

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