DE 25

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fallguy1000
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Re: DE 25

Post by fallguy1000 »

dennisb wrote: Wed Dec 22, 2021 11:51 am If I take the DWL weight for the 25' version, 2800 Lbs and multiply it by 1.331 I get a 3727 Lbs for a new DWL weight, the boat should have the same draft (scaled). Is this logic correct?

No. The larger version boat will have 121% the wetted surface area (roughly). And thanks to Phil for correcting me.

If you add 133% of the weight to 121% of the area; she's gonna be lower in the water. This is one of the reasons I am not a fan of scaling in all directions.

See what the other guys say. I am an amateur here.
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Cowbro
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Re: DE 25

Post by Cowbro »

fallguy1000 wrote: Wed Dec 22, 2021 4:42 pm
dennisb wrote: Wed Dec 22, 2021 11:51 am If I take the DWL weight for the 25' version, 2800 Lbs and multiply it by 1.331 I get a 3727 Lbs for a new DWL weight, the boat should have the same draft (scaled). Is this logic correct?

No. The larger version boat will have 121% the wetted surface area (roughly). And thanks to Phil for correcting me.

If you add 133% of the weight to 121% of the area; she's gonna be lower in the water. This is one of the reasons I am not a fan of scaling in all directions.

See what the other guys say. I am an amateur here.
Dan,

It isn't 133% of the weight to 121% of the area, it is 133% of the weight to 133% of the volume below the Designed water line. So it should work out just fine. And yes, if the original draft was 10" the new DWL would be at 11".

My $.02 on this matter is to not worry too much about it. Build the boat efficiently and have a plan to add ballast as needed if necessary. I think it will all work out okay. The weight of the boat should grow 21% just with the extra material not including any extra layers of glass.

I will say that a 90-115hp OB seems a little small for a 27.5' boat even one with only 6* deadrise. I think the extra 100lbs for a 150hp would be well worth it and probably wouldn't affect fuel economy much. I'd rather not be running full throttle just to get to cruising speed.

Phil

Edit: I would be curious what Jacques' input would be on this.

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BarraMan
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Re: DE 25

Post by BarraMan »

When I scaled up my build by 10%, I didn't give this stuff any consideration at all! 8O Way too technical for my simple, or ignorant brain! :doh:
Seems to have worked out just fine. :lol:

I really like the DE25 and if I was 10 yrs younger would be tempted to build one. :D I too would scale it up 10%, and I would hang a Yammie 200 on the back because I like a bit of grunt when required! :lol:

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Re: DE 25

Post by fallguy1000 »

I agree the 150hp seems a good size for the De29.

TomW1 can probably do some estimates on prop and speed for any given weight estimate.

Most boats given bottom paints to stay at sea on a mooring are painted 3" above dwl. I am going 5" on the Skoota.

Hull immersion is measured in kg/cm or pounds per inch. Without going into the derivation, it is typically waterplane times 5.333 for seawater, 5.2 fw.

Original boat ppi
600#/in
/5.2
=115 sqft (this is waterplane, not wetted surface), so this is a knifecut at waterline
115*1.21= 139.6sqft (bigger boat)
139.6*5.2= 726#/in.

By the way, the waterplane is one of the main numbers in hydrostatics..

Displacement is the bit of volume below that line.

The boat picks up 726# of displacement for the inch she sinks lower. So I get 2800 + 726 = 3526# for the larger boat versus the 3724. And, mainly, with Phil here, the caution is always getting too heavy. If you find you are light; you can just epoxy resin lead shot 25#s here or there.
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rick berrey
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Re: DE 25

Post by rick berrey »

The DE25 study plans have 25 mph with a 90 at 2800 disp , 22mph with a 90 at 3600 . The DE29 reports 23 mph with a 90 , that is a good number with 4 extra feet . Either he was able to keep the boat light , below 3600 , the skinny hull made a difference , or the number,s on the study plans are conservative . A 115 is showing 26 mph at 3600 disp. , based on the DE29 I would think if a DE27.5 were held down to a 4000 disp or below to Fall Guy,s math , a 115 would put it at between 26 mph and 28 mph , close to the 30 mph goal .

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Re: DE 25

Post by fallguy1000 »

rick berrey wrote: Thu Dec 23, 2021 11:53 am The DE25 study plans have 25 mph with a 90 at 2800 disp , 22mph with a 90 at 3600 . The DE29 reports 23 mph with a 90 , that is a good number with 4 extra feet . Either he was able to keep the boat light , below 3600 , the skinny hull made a difference , or the number,s on the study plans are conservative . A 115 is showing 26 mph at 3600 disp. , based on the DE29 I would think if a DE27.5 were held down to a 4000 disp or below to Fall Guy,s math , a 115 would put it at between 26 mph and 28 mph , close to the 30 mph goal .
When you calculate; you really ought to consider 3/4 throttle. WOT is always pushing the limits...

just a friendly remark...if you calc 27 mph wot, then more practical speed at 3/4 throttle is going to end up around say 20 mph cruise. I have forgotten where Dennis intends to run this boat...but figured I'd say something...
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TomW1
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Re: DE 25

Post by TomW1 »

Remember when you scale up by 10% in all directions you must scale all frames by 10% in width. This will add a small amount of weight, say 3%. The weight of the 10% addition on the stern will add the weight of the wood you use. A simple calculation of adding the weight of each piece. This only applies to the weight below the deck. Remember most of the added 10% is air. As far as motors I would go with the the 115 for the stretched boat it only weighs 20+ lbss more that the 90 but will give you more control in heavy seas and currents.

The new weight of the hull should be no more than about 2050-2150lbs, with the added wood and fiberglass, instead of the 1850lbs listed.

Since you have added volumn in all dimensions of the current dimensions a new PPI needs to be calculated. Displacement has changed and PPI has changed. It is not the simplistic calculations used by Cowbro and fallguy. They do not take into account the 10% increase in the other frames, the change in displacement or the change in PPI by the overall widening and lengthening.

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

fallguy1000
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Re: DE 25

Post by fallguy1000 »

Unbelievable.
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dennisb
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Re: DE 25

Post by dennisb »

I have a question concerning the DE25 drawings, specifically of the transom. On drawing E270/4, are the dimensions the actual for the transom height, or do I need to adjust the height to account for the 15 deg transom angle.

Thanks in advance

Dennis

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Re: DE 25

Post by Cowbro »

Dennis,
I don't have plans for the De25, but I have plans for about 5 other boats from here and all of them show the 'cut-to' dimensions for the transom, no extra calculations should be required.

You can double check it by measuring rhe length along the end of the side panel and see if it matches up with the height of the adjacent part of the transom, they should be nearly identical. If that makes any sense.

Phil

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