What Speed & Fuel Consumption

[Gludy]

As a novice, I hope you do not mind me putting some questions to you all in the hope of me trying to learn from your experiences.

I am currently at the stage of learning about boats prior to a purchasing one when I feel I know enough.
Here are my questions:-
Considering, as an example, a 33 footer planing hull (eg Princess or similiar):-
1. What size of twin diesels are the best for such a boat?
2. At what speed does such a boat begin planing?
3. What is the fuel consumption at various cruising speeds?
4. Is it possible to use more fule at lower than optimum speed?
5. Basically what are the advantages and disadvantages of the variuos size engine options on such a boat as regards both performance and fuel consumption?

I know these may be basic questions but answers are very difficult to come by.
Kind regards
Paul

 

[hlb]

The questions are academic, because unless your thinking of a new boat., you will just have to go along with the choice of available options.
You need 50hp per ton of boat as a minimum,
The boat will start to plane about 14 knots.
A Princess 33 is usually fitted with 130hp diesels, will do about 18 knots and cruises about 16.
Expect one mile per gallon then everything else is a bonus.
The rest if far to complex to go into here and will vary wildly dependant on sea, hull and other conditions

Haydn

 

[stuartw]

Try Google.com

I have just done a search for the very questions you've asked by using Google.com search engine. I put into the search field "boat fuel consumption+planing hulls". This gave a huge list of websites, some forums, some commercial and some academic. One in particular was on:-
www.bruce-roberts.com/public/HTML/POWERBOAT_DESIGN

It was very informative. So the answers are out there:-
good luck
Stuart

 

[Deleted User YDKXO]

Agree with Haydn but you might get a bit more than 1 mpg, say 1.25mpg for a 33 footer but for cruise planning purposes, you should work on 1 mpg to be safe or at least until you know your boat well. The fuel consumption will increase in heavy seas, particularly if you are throttling off/on as you hit waves.
Once you are on the plane, mpg will not vary dramatically with speed but, obviously, it will be worst flat out. Your boat speed will probably be more dependent on sea conditions in any case and engine manufacturers recommendations - Volvo, for example recommend max cruising speed as 200rpm less than max rpm (note that max rpm will vary with load, how clean the hull is and engine condition and may be less than the manufacturer states in the manual especially for older boats).
Yes it is possible to use more fuel at lower speeds. Some boats have a pronounced 'hump' between displacement and planing modes (you will recognise this when the boat is very nose high and the stern squats in the water - usually around 10-15 knots) and fuel consumption may be worse at hump speed than at higher speeds.

 

[petem]

There are other factors to consider such as saleability (bigger the better), engine noise (bigger engines may be quieter). Also that bigger engines won't necessarily cost much more to run. And most importantly make sure they're diesels (30p per litre instead of 90p).