# Design Question on powering

Discussion in 'Boat Design' started by Salmoneyes, Oct 4, 2018.

1. Joined: Sep 2018
Posts: 89
Likes: 1, Points: 8
Location: Southern Oregon

### SalmoneyesJunior Member

She is Bruce Roberts penned but strayed a bit. Keel was lengthened, the masts lowered with smaller sail plan, and she is too heavy.

Hull was built with a technique I was unfamiliar with but common for commercial steel fishing boats in Sweden. Can be modded to be stronger and loose weight at the same time even if only a 100 pounds.

I have access to help from Bruce but we are looking at many options to get her numbers to all jive, and I am trying not to have him redesign it multiple times.

The entire propulsion system is one way I possibly can drop 900 to 1000 pounds.

2. Joined: Sep 2018
Posts: 89
Likes: 1, Points: 8
Location: Southern Oregon

### SalmoneyesJunior Member

the disp/length ratio came out to to 447 using tom doves calculator.

I looked at the formulas and I am working it out on my own, but I am teaching myself the math as I go, so it is a very slow process. When I start getting a headache I go work on the boat demo..
Unfortunately, im running out of things I can do without having a master plan which Im making, thanks to help from this forum...

3. Joined: Oct 2008
Posts: 7,129
Likes: 1,070, Points: 113, Legacy Rep: 2488
Location: Japan

Ive no idea what/who Tom Doves calculator is.

It is a very simple calculation.

The waterline length - in metres
The displacement - tonnes

The LD ratio is thus: Length / (displacement)^1/3

That's it.

4. Joined: Sep 2018
Posts: 89
Likes: 1, Points: 8
Location: Southern Oregon

### SalmoneyesJunior Member

Sorry, I was referring to Gerrs formulas found in the Prop Handbook... There was more to that book then I thought originally.

5. Joined: Nov 2012
Posts: 811
Likes: 62, Points: 28, Legacy Rep: 41
Location: Delta BC

### JSLSenior Member

Displacement Length ratio in imperial is:
D/(0.01 x LWL)^3
D = long tons (2240#)
LWL = feet
as noted in Gerr's book pg 13, and also several other books.
With your Disp/LWL ratio of 440+ you might expect a top speed of around 6.5 knots under power.

Last edited: Oct 29, 2018
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.