one prop shaft or two?

By the prop calculator that I use, 1200 lb boat with 150 HP spinning a 11 X 19 prop will get me around 77 MPH.
What size props would you need for twin screw? My chart doesn't figure for 2. I'd like to go with the twin screw.

 

Divide the engine power by 2 (that's what goes to each prop) and redo the calculations.

 

I've done the calulations all the same except that the HP is now at 75. The speed is now 54 MPH and the prop is 9 X 14.
Does this now mean that if I use the 9x14 props on a twin screw, I will get back the same MPH I had for a single screw at 150 HP? Damn,,,that's easy.

 

Looks like your prop calculator tries to find out the max speed with 75 HP (which is possibly why the top speed has decreased) and calculates the propeller for that speed. What software do you use?

 

I using software that I found on the Castle Marine site. They're in Caernarfon, UK.
I've used this to find a prop for my surface drive hydro. Hope it's a "close enough" chart.

 

the problem with two props off one engine become they are not balanced like counter rotating props... there is no benefit to two over one in this case i.m.o.

-if you use surface peircing- then you would have to catch air anyway. so your props would be cupped.

and you will lose some efficiency too with twins even spinning at the same rotation.
is the issue that you dont have the space for a single in the main hull?..whats your design proposal?..hydo?..tunnel? stepped?

 

i have anew computer and it doesnt run castle marines anymore...i liked their program...try vicprop...

 

The hull style I have in mind would be a stepped tunnel. I have worked out (in my head) a way to counter rotate one shaft. The reason for one engine is weight and room in a 14 footer. I just might go with the single screw to lessen the "breakage" possablity.

 

Great! i would be veryinterested to see how you solved that--one problem with counter rotating props is if you design a second prop to counter rotate - the system that you design will be assymetrical in forces..that is- for example- lets suppose in a boat you used two ccr engines and one of them you reversed to run backwards, giving cr...this propblem then becomes wieight shifts , the engine and torque on the hull is completely different than the other, because of the arrangement of the engines- on one you will need some sort of v-drive--this mujcks everything up..ESPECIALLY if your into high performance running..
now lets suppose on one of your shafts - you simply added a device such as a pully or belt or whatever to spin one prop opposite--now the forces are not balanced out because one system will have differing friction co-efficients, rpms differences, torque differentials etc etc and this will pull the whole symmetry of the system into chaos...the vibrations form this might even break the boat apart if those dfifferences were severe enough...

however havign said all that--you have to do what you think is proper. personally id go for a single..or use to proper cwr-ccwr to get the right "tunings" for the boat... a single prop is more efficient...but two spaced far enough apart would be great too......in the end for me i try to keep it simple as possible...

 

Also, it wont cancel the motor torque twist. Neither will contra-rotating mind you.

Is this something your wallet has considered?

-Tom

 

My design involves mostly auto parts from a scrap yard, some aluminum pieces, lathe work, and welding.
I build the boats myself from plans. Anything else usually comes from E Bay.

 

ebay is good for deals-

if u get chance maybe u could post a diagram of your proposed set-up...id really like to see how it might be done? won't criticize it-scouts honour!- maybe itll work fine..but perhaps we can help fine tune it a bit?
i did wonder: maybe you could add transoms-and use two outboards? this way you could use two smaller engines to create more hp? each transom on your sponsons could have one...costs might not be too bad...of course racing enignes r big bucks..
if your using a standard engine- u might not get enough revs to use surface piercing props?- i believe they need high revs...what rpms r u planning to run at?

 

The revs all depend on what engine I find. I was thinking of a Quad 4 Olds engine. It has 150 stock HP and can rev to 5400. Other engines would be overhead cam asian types. By using belts, there is a possiblity that the props could be overdriven, but that inturn would cause more vibration. I have a sketch of the layout, but it was to be in pixels to download. I'll work on that.
Two outboards , to me, would cost more than one inboard. I live on a lake where outboards hold their price.

 

One engine -one prop, two engines two props. If you want speed, keep it simple, light and just what you need. Keep the weight down, no dual batteries, two rudders, dual fuel tanks and all kinds of stuff you don't need.

 

Im inclined to agree with mydauphin- the k.i.s.s. principle..always has worked for me...got me on the water faster- more enjoyment because less parts to go wrong...
light- efficient- and the bottom line is to enjoy your ride-- unless of course you like the engineering aspect. some times it is necessary to be complicated but 90% of the time--its common sense approach that works.

* there are only a couple "practical" ways i know of to get twins with one engine- 1. hydraulics--too heavy and not fast enough- for you-completely out of the question 2. electrics--possible but would be experimental and complicated. the batts are what will kill you-- you could use small electirc motors with high rpms -true. but one charge of about 300 small "d" cells wont go far plus its expensive to find 300 rechargable lithium ions...i looked into electric drive for my tug- but gave up the idea because it was too complicated... good luck --keep us posted on your decision...