DE 27.5

Power Boats only. Please include the boat type in your question.
jacquesmm
Site Admin
Site Admin
Posts: 28215
Joined: Wed Aug 14, 2002 1:00 am
Location: Florida USA
Contact:

Re: DE 27.5

Post by jacquesmm »

Thanks Fallguy for bringing this to my attention: I missed this thread.

First, be careful with those estimations. Always ask me (the designer) to give specs, don't guess. I can say double the layers as a first approximation when we talk feasibility but once a decision is made to build, let's finalize the BOM here or per email.
The DE25 was designed as a light boat for moderate HP and over the years, builders have built them much heavier fitted with much more HP.
In addition to the extra HP (and speed), some have scaled the boat up.
Ideally, we should use thicker plywood on the bottom (1/2") and more glass. Unfortunately, anything thicker than 3/8 will not bend easily.
We can increase thickness with more glass but it is expensive and not as efficient.

I don't remember if this is a scaled hull or a stretched hull. I remember that we talked about it but did not see the posts from Sunday.

The last BOM, one that already took in account more HP than the original design, shows 2 layers 12 oz. biax in and out. The old BOM was one and two layers, that may be where the doubling comes from.
Two layers each side is more than enough if the boat is kept at semi-displacement speeds and designed weight. My design goal was to get a 1/2" thick bottom panel: 3/8 plus the two glass skins.
If the boat is built scaled up, heavier and uses more HP, let's say more than 125 HP or more than 25 mph. we could use 3 layers each side. A 3 layers skin is 36 oz.
To keep adding plain bias may not be the best way to go.
There is another, better alternative. Since we aim at thickness, we could use bias with mat. That gives us thickness at lower cost in glass (not in resin) and better fiber orientation. If available, we could have 2 layers of 1208. If not, one layer bias 12 oz. and 1 layer 1708.
The theoretical thickness of 12 oz. is 0.021", 3 layers = 0.063. That's theoretical, amateurs will get almost double thickness or a total for the two skins of 1/4". 3 layers will work but stitch mat is even better.
Considering what is available from BBC, we can do better with 1 layer 1708 plus 1 layer 12 oz. The strength is more than sufficient but thickness will be 0.054 + 0.021 = 0.075 in theory, 0.15 per skin in the real amateur builder world. That is what I recommend in this case unless there is some factor, weight or HP that I overlook.
The 3 layers skin is 36 oz., the 1708 + 12 skin is 37 oz. but thicker.

I would also increase the tabbing of the bottom framing by 50%. Where I specify 2 layers, please use 3.

In a nutshell, either use the original scantlings multiplied by 1.5 or better, use 1708 and 12 oz., one layer each.

And let me know if this is a stretched hull or a scaled one and what the HP will be.
Jacques Mertens - Designer
http://boatbuildercentral.com

Online
rick berrey
Active Poster
Active Poster
Posts: 382
Joined: Sat Jun 20, 2009 2:16 pm

Re: DE 27.5

Post by rick berrey »

I think the builder of the DE29 did use 1/2" ply on the bottom , I don't remember off hand but I also think he widened the beam by about 8" , so he did get the ply to bend somehow .

fallguy1000
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 10198
Joined: Tue Jan 12, 2010 12:25 am

Re: DE 27.5

Post by fallguy1000 »

Just an adjunct to JM I did this morning to check my gutcheck for you Dennis.

Using Gerr's scantlings which are generally excessive.

Beam

8.3 x 1.1 = 9.13 '

Length

27.5

Depth of hull?

3 x 1.1 =3.3'

Scantling # (SN)

9.13 x 27.5 x 3.3 /1000 = 0.83

Adjustments for odd hull shape - none

Solid skin(all glass) calc lower topsides

= 0.25 x cube root SN (0.94) =0.235"

Bottom laminate

1.15 x 0.235 = 0.27"

D/L ratio check

Displacement 2800 x 1.33 = 3724
Length 27.5.
D/L = 135.4 -> not light ship, not heavy ship

Foam cored construction adjustments

foam cored hull thickness

= 2.2 x 0.235 = 0.517 total thickness

Skin recommendation per Gerr for foam

Outer
0.4 x 0.235 = 0.094"
Inner
0.3 x 0.235 = 0.071"

These are for roving/csm, stitched glass is thinner.

For strip planked vessels..heavy calc for boats with HIGH SN of >2, keep in mind I came up with 0.83 SN

Hull fabric exterior
30.8 + 8.3 x 0.83 = 37.689 oz

Hull fabric interior = 66% of 37.689 or 24.87 oz

These are close to what I recommended (intuition only) and I said they were excessive and they are as they are recommendations for larger, faster vessels.

If you use 1708, and db 1200; it would be two layers of work each side. 1708 must be bottom wetted or epoxy on substrate, that is. It is harder for amateurs, but totally doable...

Not sure why JM didn't mention 1808 for 0/90 fibers, but also an option to replace the 1708, long strands out.

Another option, away from the designer's advice, mind you would be to run db1200 on 45 degree angles, so you'd get some 90 tows. Overlaps are required and this makes fairing very difficult because you don't want ridges on the bottom for drag. You would do so if u had tons of db1200...which can happen, but again, you must fair to flat which means the 0.025" ridge is a hole in the middle of ever layer of glass and must be filled for professional quality work. Outer strands should run the length of the vessel, not the short way..

Bottom line 36 oz or so out and 24 oz or so in. How you get there is to follow JM or ask if you want a different layup..

I believe he is scaling all 3 directions Jacques.
My boat build is here -------->

viewtopic.php?f=12&t=62495

TomW1
Very Active Poster
Very Active Poster
Posts: 5844
Joined: Sun Apr 19, 2015 10:57 pm
Location: Bryson City, NC

Re: DE 27.5

Post by TomW1 »

Jacques I am a little surprised by your answer. Normally when a builder stretches a boat by only 10% which he is doing 25 x 1.10 = 27.5 means only an extra layer of 12oz biax on the bottom and an extra layer of 6 oz on the seams as long he keeps the motor within reason, in the lower range of HP. I would say under 150 which is plenty of power for this boat.

Comments?

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

jacquesmm
Site Admin
Site Admin
Posts: 28215
Joined: Wed Aug 14, 2002 1:00 am
Location: Florida USA
Contact:

Re: DE 27.5

Post by jacquesmm »

Tom, a 3rd layer of 12 oz. was my first option, a 1708 plus one layer of 12 oz. is almost the same glass weight but offers some slightly better fiber orientation.
Fallguy, Gerr's calculations for foam take in account a thicker core and less glass but he ends up with a panel of the same total thickness than mine.
His strip core scantlings are very close to mine, except for the inside skin which is a little lighter.
I assume that Gerr calculates for professional building. For amateur building, I prefer to be on the safe side and use a larger safety margin. Not that large, one less layer will probably be good enough in 90% of the cases but I prefer to add one layer.
The original scantlings show skins of 24 oz.. I propose 36 or 37 oz. = 50% more. I can't go in between, I can't spec a half layer.

It is fun to discuss all this but our builder has to make a choice between 24 or 36 oz., 2 layers or 3 layers. I say 3 to be safe.
Jacques Mertens - Designer
http://boatbuildercentral.com

dennisb
Frequent Poster
Frequent Poster
Posts: 81
Joined: Mon Jun 28, 2021 2:45 pm
Location: Connecticut

Re: DE 27.5

Post by dennisb »

All, Thanks for the comments and guidance.

I have been traveling on business this week, and just got home, thus the reason for the slow response.

I have stretched the boat by 10% in all directions. I have also used 3/8" plywood for the bottom.

I need to digest the recommendations for glassing the bottom, will do that this weekend.

Again, thanks for the comments.

Dennis

dennisb
Frequent Poster
Frequent Poster
Posts: 81
Joined: Mon Jun 28, 2021 2:45 pm
Location: Connecticut

Re: DE 27.5

Post by dennisb »

Sorry for the additional post,
My plan is to use a 115 HP motor. I believe this will give me the performance I am looking for.

fallguy1000
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 10198
Joined: Tue Jan 12, 2010 12:25 am

Re: DE 27.5

Post by fallguy1000 »

dennisb wrote: Thu Apr 21, 2022 7:08 pm Sorry for the additional post,
My plan is to use a 115 HP motor. I believe this will give me the performance I am looking for.
Hey Dennis,

My only aim is to help, but I am a little worried you will be unhappy with the 115hp engine.

I forgot if Tom ever did a workup, but the 90hp is the bottom engine for the de25, so your boat is 133% bigger and 133% of 90 is about 120hp... Most boats get bigger and adopt more load...cabins get bigger, more gear, more headcount... I think you ought to be considering the next logical step up in the 150hp class of engines.. At least see what happens with the boat fully loaded say with 6 passengers and gear and if it'll get up on plane. TomW ought to be able to run the numbers. I'd use fully displacement weight plus 1000 pounds to get to a bunch of pax and a headwind scenario.

My 50 horse has trouble putting my 16' boat up on plane if the deck gets wet, 3 big guys and a stiff breeze/chop headwind...so I really am worried the 115 is undersized by about 20%...The DE27.5 is more than 2x my skiff.

Challenge the idea a bit more...the 115 would be fine flat water, but pretty sure she is gonna dog in the hole loaded into a 18 knot wind...
My boat build is here -------->

viewtopic.php?f=12&t=62495

fallguy1000
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 10198
Joined: Tue Jan 12, 2010 12:25 am

Re: DE 27.5

Post by fallguy1000 »

Patrick's DE29 is only running a 90hp.

So, I might be off. But I run in big water here on Lake Superior, so perhaps my bias is coming through.

115 should be fine unless u run in big seas
My boat build is here -------->

viewtopic.php?f=12&t=62495

jacquesmm
Site Admin
Site Admin
Posts: 28215
Joined: Wed Aug 14, 2002 1:00 am
Location: Florida USA
Contact:

Re: DE 27.5

Post by jacquesmm »

Somewhere on this forum, there is a long thread about a DE25 built heavy for which we hesitated between a 115 and 150 HP.
The builder picked a 150 and is happy with it.
This particular boat was built for cruising with a family, lots of gear and diving equipment. If that is the plan, go for a 150.
On the other end of the scale, a minimalist DE25 will move at semi displacement speeds with a 50 HP but as said on the study plans, I recommend a 90 or more.

It all depends on the displacement (total weight).
The hull length is not a major factor, the weight is. You can see that in the table shown in the study plans.
The Crouch formula that I use does not use the length but the weight. That formula has always been very accurate for my designs, TomW uses the same formula.
I would pick a 150.
Jacques Mertens - Designer
http://boatbuildercentral.com

Post Reply

Who is online

Users browsing this forum: rick berrey and 7 guests