Beam at waterline

[Snowgoose-1]

Prop calculators normally request this variable.

Is there a rule of thumb for a .modernish hull ? They best I can find is 10% of
max beam which seems low


Ta

 

[Roberto]

Rather 80-85%
BWL and canoe body draft are usually taken to approximate midship area hence approximate the prismatic coefficient hence the type of relative speed the boat is designed for; you should be able to find different formulations using just displacement and type of boat.

 

[mjcoon]

Prop calculators normally request this variable.

Is there a rule of thumb for a .modernish hull ? They best I can find is 10% of
max beam which seems low


Ta

Surely that must be 10% less than max beam, viz 90%

 

[Marsali_1]

I just did a couple of quick measurements on some builder's drawings for our boat and the maximum beam is 105" +/- at the deck level and the waterline beam at the same station is 94" +/-. That works out to about 10% less than the max. Maybe that is what is meant by the figure you found?

 

[Tranona]

I normally use the 80-85% suggested earlier. Not sure it makes a huge amount of difference. Just run the figures for my boat using 80% and 90%. No difference in calculated speeds and propeller size.

 

[Snowgoose-1]

Surely that must be 10% less than max beam, viz 90%

Yes, apologies. That's what I meant. Hadn't fully woken up from my afternoon sleep.

 

[Daydream believer]

Yes, apologies. That's what I meant. Hadn't fully woken up from my afternoon sleep.

I hope that you woke up before you went sailing down the Orwell, with all those inconsiderate sailors tacking their way to Ipswich