Minimum propeller centre distance?

[PetterM]

Looking at a long very slender monohull with twin propellers, on this particular design the optimum propeller diameter meter is limited by having sufficient tip clearance between the propellers.
It there any rules of thumb how close I can put the propellers?

[Ad Hoc]

If the flow into the props is identical and clean and they are basically inline....shouldn't make any difference

Originally Posted by PetterM

Looking at a long very slender monohull with twin propellers, on this particular design the optimum propeller diameter meter is limited by having sufficient tip clearance between the propellers.
It there any rules of thumb how close I can put the propellers?

Did not understand clearly. Do you ask for tip clearance to hull or between props?
To hull is min 15% of prop diam. (better 20), between props is min. 1,5 prop diam. better 2. On a very slender hull, you will have a problem or you use a deadwood.

[PetterM]

Thanks for your replay.
I should have expressed myself clearer. What I meant to ask for is the tip to tip clearance. So minimum .5 of propeller diameter?

Originally Posted by PetterM

Thanks for your replay.
I should have expressed myself clearer. What I meant to ask for is the tip to tip clearance. So minimum .5 of propeller diameter?

1,5, read: 1,5 or deadwood!

[PetterM]

What would happen if the propellers were mounted too close? Is there any literature on this?
The propellers are designed to run relatively low slip at design point.
Thanks,
Petter

[Ad Hoc]

Not much literature that I know off. However having designed several boats in the past, with props all in line and very close together (ie just several cm's apart tip to tip) no losses were noted boat did the expected speeds and all data as predicted on sea trials. It is of course a different matter if the prop's are offset against each other.

[Frosty]

I agree with ad hoc if the flow to the props is clean it shoud'nt make a difference. However the whole point of having 2 props is lost if it is for maneuverability.

[Ad Hoc]

Good point Frosty, manoeuvrability comes into play too...unless the boat is not meant for slow speed manoeuvring.

Beg's the question then..why have 2 that close, any particular reason?

I´m happy to agree with Frosty (thats quite rare) in case of maneuverability. The distances given are (as asked for) rules of thumb, above 1,5 prop diam. there are no measurable rotational interferences which CAN occur definetily below such distances under certain conditions. I can´t find the papers, but am sure to recall "Hucho" and "Fresenius" on that subject. Maybe John has? I know "Hamilton Sundstrand" once made some related analyses on air props.

[powerabout]

on a race boat they could be as close as 30mm tip clearance or less.
You didnt mention fast or displacement.
Under the hull, surface drive, stern drive or what or the diameter or prop rpm

[Ad Hoc]

Apex/Richard

I'll look through my old files on prop's. I know I have lots of old research reports from VT, KMW, US Navy and others, but not sure about those you have mentioned. Need to get my duster out...they are old papers!!

Originally Posted by Ad Hoc

Apex/Richard

I'll look through my old files on prop's. I know I have lots of old research reports from VT, KMW, US Navy and others, but not sure about those you have mentioned. Need to get my duster out...they are old papers!!

Archive of applied mechanics (here 4th Int. congress for applied mechanics 1935 Cambridge, Flügge, Lotz et al.)

[CDK]

Is it not the engine width, plus mountings etc. that determines the distance between props?
Unless you have some sort of Siamese twin engine, imho the prop size is always smaller than the required engine space.