Can a cathedral hull move at displacement speeds economically?

[Johnn]

I would like to know because I bought my boat mostly for the deck space, and I wont be planing alot, just trying to find the lobbie pots! What type of engine would do? (17 ft)
Thanks

 

[duncan]

I would like to know because I bought my boat mostly for the deck space, and I wont be planing alot, just trying to find the lobbie pots! What type of engine would do? (17 ft)
Thanks

it could be argued that the hull shape isn't a factor in fuel consumption at displacement speeds - and then again the cat brigade would argue it's everything!


on balance, in the way I think you are asking the question, the answer is 'it won't matter'

 

[omega2]

We had a pilot 590, the main was a 90horse evinrude, but the aux was a seagull plus 40, 4 horse power, and that could move us along. One things for sure big two strokes do not like pottering. Maybe 25 horse four stroke would do you.

 

[Johnn]

We had a pilot 590, the main was a 90horse evinrude, but the aux was a seagull plus 40, 4 horse power, and that could move us along. One things for sure big two strokes do not like pottering. Maybe 25 horse four stroke would do you.

Thanks for the reply, is it true that if you keep the throttle around 40% its the most econimcal?

 

[duncan]

Thanks for the reply, is it true that if you keep the throttle around 40% its the most econimcal?

not necessarily because 40% could put the hull into the hump area etc etc etc - and that's assuming you mean 40% revs and the boats properly propped

 

[Chris_d]

Max displacement speed of a 17footer of any type will be about 4knts, which will only need about 4hp to achieve it, if you just want to potter about and use less than 0.5gph with power in reserve 15-20hp will be more than enough.

 

[Johnn]

Thanks for all the replys, I am thinking of putting a 30hp 4 stroke on it, would this cut down on fuel economy, as I would have more speed than the other outboards at lower throttle?

Also would the hull shape, (cathedral) have any dramatic effect on my fuel consumption?

Thanks

 

[nyx2k]

i used to have a 18ft with cathredral hull it was a birchwood something and 5hp would push it against the flow on the trent and 20hp would get it planing so anything around 20-30hp would be ideal i think.

 

[Nautibusiness]

Max displacement speed of a 17footer of any type will be about 4knts, which will only need about 4hp to achieve it, if you just want to potter about and use less than 0.5gph with power in reserve 15-20hp will be more than enough.

I agree or almost do anyway.

If the boat is only ever going to be run at displacement speed then 10 to 15 Hp will be plenty, but resist the urge to push the throttle beyond about 1500 to 2000 rpm.

Push the throttle forward to gain the extra knot or two and you will start to use a lot more fuel.

 

[Chris_d]

I agree or almost do anyway.

If the boat is only ever going to be run at displacement speed then 10 to 15 Hp will be plenty, but resist the urge to push the throttle beyond about 1500 to 2000 rpm.

Push the throttle forward to gain the extra knot or two and you will start to use a lot more fuel.

Agree, I probably didn't make my point very well, the op asked if a 17ft dory would be economical at displacement speed, but I suspect he is thinking displacement speed means up to about 10knts, as the displacement speed of any boat is a function of its waterline length to do 10knts in 17footer it will be either just planing or burning loads of fuel. So his answer is if he wants to cruise at say 4-10knts it will not be very economical because its above the boats displacement speed discounting the hull shape. However at these intermittant speeds a dory will probably be a bit better than a deep vee planing hull, but not as good as semi displacement hull.

Think its splitting hairs though, any engine between 10 and 50hp will give much the same result at such low speeds, i'd just go for the biggest it will take

 

[Johnn]

Hi,
I have heard about the 'slam' on rough/chop seas for cathedral hulls, could anyone tell me the slam is only when your at speed? , does it happen at real slow speeds?
Thanks

 

[AndieMac]

John, if you are setting and retrieving pots in open coastal water, my concern would be more vessel stability and safety should the conditions deteriorate, especially during the retrieval process, as these are the times when folks tend to put themselves (and crew) at risk.
If the pots have been in overnight and you are going home that day, the pressure is on to get them back, even if the sea conditions are marginal.
The little boat will not feel safe pootling along at D speeds with cresting waves abeam or astern of you.

Make allowances for the additional weight of the normal crew and the pots and associated gear, so you can run if you need to.

 

[Kawasaki]

A Mac has obviously done the Job Johhn and has made some good points.
Most lobster guys around our area use monohulls (like Orkneys etc) that can stand a bit of a chop.
The cathedral hull is good when all is flat and very stable at rest.
OK they can take a bashing when running into a head sea but thats what you will get a 'bashing'!
So take that into consideration
As others have said
You don't need a huge lump for a 17' Dory
30 ish though will get you home if you have to scurry a bit.
Tother thing a dory has against it a bit
If yer loaded with pots an gear, there aint much freeboard so remember to stow stuff properly case it does rough up a bit, else the Lobbies might just make itback home before you do!!