# 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

I have been getting great information to resolve issues on our project here, so, I was hoping I could get a lesson in the engineering process for sizing a propulsion system for a full displacement hull.

Specifically mine, as I am rebuilding the whole boat and should confirm that what we have is what we should have.

Happy to fill in blanks if you need, but i'm getting better at my math so I could give the formulas a go first to avoid wasting your time.

2. Joined: May 2008
Posts: 2,487
Likes: 280, Points: 83, Legacy Rep: 1082
Location: Beaufort, SC and H'ville, NC

### philSweetSenior Member

Gerr's Propeller Handbook is a good place to begin. It's set up as a propulsion system cookbook.

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

### SalmoneyesJunior Member

Am I using "propulsion system" incorrectly if I am referring to engine, gear ratio, prop shaft diameter and prop size and pitch?

I realize that they are all going to use different formulas but one works hand in hand with the other and should be matched..

My concerns are that as I go along with my project, I am realizing that the builders used whatever was available at the time.

For example, the plans called for up to 85HP and we have 130. The propshaft is 45mm and the screw is 23". All much bigger than commonly found on a 40 footer, however it is much heavier than the original design.

Id like to understand the math so I can make informed decisions on my mod options rather than ask the designer to run hundreds of options for me. My brother-in-law is an engineering mathematics teacher and is always happy to help me, thank god...

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

### JSLSenior Member

a 50% power increase is quite a bit. You may not need it, or be able to use it.
what is the reduction ratio, vessel displacement, LWL, etc?

5. Joined: Oct 2008
Posts: 7,115
Likes: 1,057, Points: 113, Legacy Rep: 2488
Location: Japan

In order to establish the powering requirements you need to do some other basic proprietary data gathering first.

1. Have you done a weight estimate of the vessel?
2. Have you designed or selected a hull form that matches your weight estimate?...i.e it floats on the design draft of your weight estimate
3. Have you a set of resistance curves for the hull form you have used in 1. and 2.?

Without this preparatory work, you are just guessing and have no fixed target.

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

### SalmoneyesJunior Member

I have gotten that a couple times now... I spoke with Beta folks at length and they came up with the same HP ratings on the plans (which I did not give them) so the motor we have is definitely too much. We deduced that 100hp would be max, which is lighter by a 400 lbs so we are looking at those options now...

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

### SalmoneyesJunior Member

We have the plans, and I had the boat weighed while in transit. What I was looking for was the formulas to work it out on my own (or with help from my brother in law).

I have realized in this process ( I call "rebuilding a new boat") that I need to take some responsibility of learning how to engineer things on my own, as I would prefer to ask questions AFTER working out solutions in order to confirm I am on the right track.

I think the biggest realization I have had is that even after doing the calculations on things, the answers leave room for interpretations and choices.

I am working on pilot house window choices now which led me to the forward rake. That has taken up a few days of research and I have learned that there are two strong camps on the issue. Having read soo much great stuff from each camp, I think it just comes down to trade offs.

Propulsion system design is considerably easier as the math is the math. Combining different groups of numbers allows me to get the same answer so there are choices here too.

8. Joined: Oct 2008
Posts: 7,115
Likes: 1,057, Points: 113, Legacy Rep: 2488
Location: Japan

So you have no.3 on the list?

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

### JSLSenior Member

Any photos or drawings of the boat that you can post here would help. If a displacement hull is it a power boat, sailboat, chines, round, etc ...?????

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

### SalmoneyesJunior Member

no. that is one of the formulas I have been working on... or at least trying.

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

### SalmoneyesJunior Member

LOA 44’10”

LOD 39’

LWL 34’10” . (plans say 34.03)

Displacement (wet) 42000 lbs (plans call out 26967)

Lead Ballast (11 cu’) 7813 lbs (plans say 7500 steel)

Draft 6’2.5” (plans say 5.66')

Beam @ Deck 13’

Beam @ WL 11’6”

Mercedes OM352 130 HP (plans say 40-85hp)

G0047/31 PRM 310 gearbox 2:1 1979 2:01

Prop Current (3 blade) 23” x 15

Prop Shaft Dia 45mm

SA/D Ratio 12.6

Sail Area Current 952 sq’

Plans called for 4mm steel but she was built with 6mm.
Current weight was gotten on certified truck scale during transport, and truck was weighed at same scale after offload to get the accurate weight. Weight was 37300, dry, and no standing, running rigging or ground tackle.

She is extremely overweight and we are looking at a number of options to reduce weight. Downsizing on the motor is one viable option. Beta Marine ran the numbers as they are and came up with 85hp. They offered a 105 but the cost for extra 20 ponies was not practical. Still looking at a number of combinations to get the best power (at screw) to weight ratio.

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

### JSLSenior Member

what RPM is the engine rated.

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

### SalmoneyesJunior Member

130 hp @ 2800

governed to 3200

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

### JSLSenior Member

I did a 'quick' review and
(a) That 50% increase (4mm to 6mm) in plate thickness probably did the trick re the weight matter
your disp/length ratio is around 440. A maximum should be about 350. This will affect performance and many other aspects.
(b) prop calculation looks like 22 x 15 x 3 blade - you may have to trim your 23" down a bit to avoid tip cavitation - or possibly look at a bigger reduction (2.25+:1).

Who did the design & build?

15. Joined: Oct 2008
Posts: 7,115
Likes: 1,057, Points: 113, Legacy Rep: 2488
Location: Japan