A Question For The Engineers/Naval Architects

[Mino]

Fuel Consumption: A Question For The Engineers/Naval Architects

Would I be correct in assuming that two identical mobos (with round bilged hulls) fitted with identical engines, one made from GRP, the other steel, both cruising for say 300 miles under identical conditions at 7 knots, would have a significant difference in fuel consumption; the GRP mobo burning much less due to its lighter displacement?

 

[jfm]

Would I be correct in assuming that two identical mobos (with round bilged hulls) fitted with identical engines, one made from GRP, the other steel, both cruising for say 300 miles under identical conditions at 7 knots, would have a significant difference in fuel consumption; the GRP mobo burning much less due to its lighter displacement?

Nope. The steel boat could be made with thin plate to the same displacement as GRP

 

[Mino]

Nope. The steel boat could be made with thin plate to the same displacement as GRP

Then (assuming again) would using steel because of it's sturdy reputation become null and void if one where using thin plate? Not as strong? Might as well use aluminium or GRP?

 

[PCUK]

In practical terms 4mm is reckoned to be the minimum for steel boats up to about 40feet so it would be heavier than the GRP equivalent and would use a little more fuel, but not a huge amount being displacement.

 

[jfm]

Then (assuming again) would using steel because of it's sturdy reputation become null and void if one where using thin plate? Not as strong? Might as well use aluminium or GRP?

Just to be clear, i was answering your question precisely, which was "Would I be correct in assuming...". I'm saying you should not, at least in general, assume it; you should get the facts. The steel boat could be heavier, lighter or of equal weight. Overall though, you have given us very little hard info to work on!

A typical say 15m 4mm plate steel boat will be heavier than a 15m GRP boat. If you built the boat from lighter steel plate then as you say its reputation for sturdiness and easier repair wouldn't apply any longer. But you could have a thick steel hull and then fit a £2m carbon fibre superstrucure, with cabin linings in expensive ply-foam-ply sandwich, and even carbon fibre toilets, and you might then get the weight equal to a 15m GRP production boat

So it kinda depends exactly what you're asking...

 

[Mino]

Just to be clear, i was answering your question precisely, which was "Would I be correct in assuming...". I'm saying you should not, at least in general, assume it; you should get the facts. The steel boat could be heavier, lighter or of equal weight. Overall though, you have given us very little hard info to work on!

A typical say 15m 4mm plate steel boat will be heavier than a 15m GRP boat. If you built the boat from lighter steel plate then as you say its reputation for sturdiness and easier repair wouldn't apply any longer. But you could have a thick steel hull and then fit a £2m carbon fibre superstrucure, with cabin linings in expensive ply-foam-ply sandwich, and even carbon fibre toilets, and you might then get the weight equal to a 15m GRP production boat

So it kinda depends exactly what you're asking...

Fair point! As you are aware, my extremely limited knowledge has produced a rather 'man on the street' question, but you and PCUK have managed to answer it most satisfactorily! Thank you, gentlemen.