Transom structural calculus

[sarciccia]

Hi to all
this is my first post,even though i've read all the posts.....they're all very interesting.
I'm an amatorial designer/boatbuilder and now i'm developing a projet of a 28 footer cabin fisherman built in sandwich epoxy vacuum bagged.
The engine will be a mercruiser i/o 300 hp.
My problem is this: I want to open a transom gate
but I don't know if the structure will be enough strong.
how can I know the load generate by the sterndrive on the transom?
the boat:
LOA: 27' 6"
B max: 9' 10"
LWL :23' 4"
BWL : 7' 4"
max Load : 7700 LBs w engine,h/w,crew,gas

Engine: Mercruiser 350 MAG MPI Bravo III

many thanks ,all answers will be appreciated

Sorry for my english

S

[War Whoop]

Work off the prop simple calculations that will give you the thrust or bollard pull then you need the leverage this force applies from the distance the prop shaft is below gimbal housing on the outside of the transom.

[sarciccia]

mmh...
It seems to be not so easy ....i'm not an engineer.
War Whoop can you help to me?

many thanks

S

[marshmat]

A very rough approximation might be described as follows:
Divide the engine power (in kilowatts) by the boat speed (metres per second) at the lowest speed where you would use wide-open throttle. The result is (roughly) the force in newtons being exerted by the prop. Multiply this number by the vertical distance (metres) from the propshaft to the centre of the gimbal housing. This is the moment exerted about the gimbal by the prop. Divide that by the distance (metres) from the gimbal to the top and bottom of the drive housing to get, respectively, the outward and inward forces on the transom at the edges of the mounting. THIS IS A VERY ROUGH APPROXIMATION ONLY! Now multiply by an appropriate (and arbitrary) safety factor to get the point load which the transom must withstand around that hole. This is not an exact method but it will give you a rough idea of what the transom needs to hold up to. Checking the engine builder's recommendations and appropriate scantling books, with the corresponding math, would be more accurate.

[wdnboatbuilder]

I have thought about this thread, and isn't there a formula in the coast guards back yard boat builder hand book? I may be wrong but it seems I have seen one somewhere in my readings. I 'll look in the book, I will warn you I have moved since I have looked at that book.

[sarciccia]

Many thanks to all,
Marshmat,I did that calculation,it results only 14,45 N at prop .
Maybe i had a mistake.
Anyway i'm planning to build my boat with this lamination schedule:
bottom
1208/0-90° 10oz/1708/1208/pvc foam 1/2" 5LBs/cf/1208/0-90° 10oz/1708
sides
6oz wov/1208/pvc foam 1/2"5LBs/cF/0-90°10oz/6 oz wov
transom
6 oz wov/1208/0-90° 10oz/1708/plywood 5/8"/1208/ply 5/8"/1208/ply5/8"/1208/0-90° 10oz/1708/0-90°10 oz
4 stringers laminated from the transom to the bow.
Each:several ply of plywood to reach 3"highx3"wide
laminated to the hull and the transom with1208/1708/0-90° 10oz/1208/1708/0-90°10oz/1208
10 floor frames:the same as stringers

what do you think?

many thanks

S

[marshmat]

Check your units. Remember the "kilo" prefix introduces a factor of x1000.
A 300 hp (224 kW) engine, if it is allowed to reach maximum power while accelerating to plane (say 14 knots, 7.2 m/s), exerts ( 224 kW / 7.2 m/s) = 31 kN. If the prop is, say, 70 cm from the gimbal, that would be (31 kN * 0.7 m) = 21.8 kN*m moment about the gimbal. Say the bottom of the mount is 20 cm below the gimbal, you now have ( 21.8 kNm / 0.2 m) = 109 kN force on the bottom edge of that cutout, as a very rough approximation.
I might suggest trying Vectorply's VLAM software ( http://vectorply.com/lamdesign/vectorlam_sw.htm ) to give some idea of how strong these laminates will be. You do need to be fairly confident in your math and basic physics abilities to make sense of its results though.

[sarciccia]

oops.....I had big mistake! Iforgot the "K"
I know vectorply's VLAM software,but I can't use it because the software doesn't run on my computer.....I don't know why.
I know that it does run under excel but the software ask for a password ....

Is there another software like it?

many thanks again

S

[buckknekkid]

very simple, ask someone who has built a LOT of boats. And how do you drag a dead tuna over top of the out drive and into the boat?

[sarciccia]

Hi Buckknekkid,
I'll use my boats for fishing and for pleasure with my family and my dog(a big black newfoundland -170 LBs).
I decided to do a transom gate on my boat exactly for pulling aboard big fishes and for go into the water easily.
I decided for a stern drive because I think that a couple of outboard will exert more load than an I/O.

Anyway, i'm not a professional builder but i love boats and any comment or suggestion would be appreciate.

many thanks
S

[buckknekkid]

I have been to Bari on numerous occasions and spent lots of times sitting by the docks. the outboards are more common in Florida but fuel and jackshafts my be good reasons to go with inboard single diesel. check out my boats in "plug made. mold made, now what " http://boatdesign.net/forums/showthread.php?t=9803 Buckk

[sarciccia]

hi Bukk,
Bari is a nice city ,next time that you'll come in Italy let me know,we'll drink a beer and why not?go for fishin' together.
I've not enough space for an inboard diesel.....
are you a professional builder?Which kind of boats do you build?Have you a website?

bye

S

[buckknekkid]

heres an interesting email I received today





SpaceAge Synthetics product comparison


Thermo-Lite Board vs Plywood

1) Can the product rot? No Yes

2) Can the product absorb water? No Yes

3) Can insects eat or damage it? No Yes

4) Accepts standard tooling & hardware Yes Yes

5) Can it be painted? Yes Yes

6) Can it crack or splinter? No Yes

7) Can acid effect the quality? No Yes

8) Can it be palletized? Yes Yes

9) Will it harbor molds and fungi? No Yes

10) Does it have a low expansion Yes No
and contraction ratio?

[sarciccia]

It's really interesting......but plywood seems to be stronger.....

S

[buckknekkid]

Quick Weight** Reference

2000 Series (20 lb/cbft)** 2400 Series (24 lb/cbft)**
1/4" 0.45 lb/sqft 1/4" 0.5 lb/sqft
1/2" 0.85 lb/sqft 1/2" 1 lb/sqft
3/4" 1.3 lb/sqft 3/4" 1.55 lb/sqft
1" 1.75 lb/sqft 1" 2.05 lb/sqft
5/4" 2.2 lb/sqft 5/4" 2.55 lb/sqft
3/2" 2.6 lb/sqft 3/2" 3.05 lb/sqft
7/4" 3.05 lb/sqft 7/4" 3.55 lb/sqft
2" 3.5 lb/sqft 2" 4.1 lb/sqft

2600 Series (26 lb/cbft)** 3000 Series (30 lb/cbft)**
1/4"* 0.6 lb/sqft 1/4" 0.6 lb/sqft
1/2"** 1.2 lb/sqft 1/2" 1.25 lb/sqft
3/4" 1.65 lb/sqft 3/4" 1.9 lb/sqft
1" 2.2 lb/sqft 1" 2.55 lb/sqft
5/4" 2.75 lb/sqft 5/4" 3.15 lb/sqft
3/2" 3.3 lb/sqft 3/2" 3.8 lb/sqft
7/4" 3.85 lb/sqft 7/4" 4.45 lb/sqft
2" 4.4 lb/sqft 2" 5.1 lb/sqft
*30 lb/cbft
**28 lb/cbft

2000 Series 2400 Series
1/2" 1/2"
17.65% lighter 2400 20.00% lighter 2600
41.17% lighter 2600 25.00% lighter 3000
47.06% lighter 3000 34.00% lighter XL Plywood*
57.65% lighter XL Plywood*
3/4"
3/4" 6.45% lighter 2600
19.23% lighter 2400 22.58% lighter 3000
26.90% lighter 2600 29.03% lighter XL Plywood*
46.15% lighter 3000
53.85% lighter XL Plywood*

2600 Series 3000 Series
1/2" 1/2"
4.16% lighter 3000 7.20% lighter XL Plywood*
11.67% lighter XL Plywood*
3/4"
3/4" 1.26% lighter XL Plywood*
15.15% lighter 3000
21.21% lighter XL Plywood*

*XL Plywood is manufactured by Greenwood Products.
3/4" weighs 2 lb/sqft.
1/2" weighs 1.34 lb/sqft.
**These weights and percentages are approx.
Board weight may vary slightly.



From experience and numerous other bulletin boards, you will find that "TRANSOM ROT" is one of the most common issues with stern drive powered boats. Whenever you drill something into it it must be absolutlely sealed.