prop size

[patrol1064]

24092010101.jpg (184.33 KiB) Viewed 4166 times

wondering if anyone thinks this is undersized.

[bearmcnally]

Hi patrol1064

What size /hp is the engine ?
If you clean the back of the prop shaft housing it should have stamped the pitch size and maker etc It looks roughly the size on Egret II I can't remember of hand her size but it's on a BMW d7 (6hp)


Cheers Bear

[patrol1064]

hi bear.i thought the sort of standard prop was 13 inch?the engine going in eventually is a 14hp.the boat is on a mooring so i cant get the size,pitch till i get it out the water,rotten winter tides are holding up the proccess.

[surrealbass]

I do have the measurement of my prop somewhere but not readily available but I think mine is smaller than yours going by pics- was powered by an Arona AD186 which i think was 8-10hp- right now there is just an empty space with a couple of dents in the bilge where my knees fit while I progressively scrape out 40 years of gunge! I am sure the prop guys will have something worked out for your new motor, just a matter if there is enough room to swing the thing- I think they use a percentage of dia for working out tip clearance.

[bearmcnally]

hi Guys

Egret II had an Arona 8hp it now lives in the garage.One of the big problems with inboards in Top Hats is cavitation So big hp engine isn't require ? Well that's what Porter's told me .

Cheers Bear

[storm petrel]

The amount of cavitation will also depend on the type of prop used and whether the prop is offset or behind the keel.
Cheers,
Mark

[patrol1064]

i didnt think cavitation would be a problem having the prop low down.what is the cause hull shape?cavitation is what put me off using an outboard.

[surrealbass]

cavitation is really air bubbles being formed at the propellor(or impellor in a pump). this occurs on a propellor usually from an excessively low pressure behind the blade, this lowers the boiling point of the water releasing the air the same way as you see it boiling water on the stove. It can be caused by a few things- including overdriving the propellor and in the Tophats case I believe because the prop. aperture is really a little small as well as the fact that the deadwood in front of the prop is really quite wide leaving not a lot of the propellor exposed to uninterupted water flow.that is why it is important to talk to a good propeller guy about the clearance of the propellor. so it is not really just about air being "sucked" from the surface ala outboard or inboard on a really scary day with lots of nasty big waves to bash into!!

[Phillip]

Hi All, I'm back on line for a few days,

I don't seem to have any cavitation problems with SEAKA. though she will sit her stern down!

I had a new prop made by I think it was AUSPROPS, or something like that.
I don't have that info at hand but I do have the prop number which is 070 1313 if thats any help.

Cost me $6-800 [figures are at home.]

Phillip
Sitting in MacKay Marina

[Tales]

A few years ago I was tempted to replace my mighty 4.5 hp outboard with an inboard.

I talked to a chap at Wichards who had fitted inboards to several Top Hats and he told me that get the best performance from the prop, one had to fair down thickness of the keel in the prop area to enable the water to get to the prop. Pretty sure I have seen a picture of a TH hull recently on this site with the fairing done.

http://www.wm-marine-diesel.com/wmmarine.htm


One of my mates down here did it when he built up his TH from moldings and with a 7 hp inboard he can out perform a normal TH with 10 hp. I’m sure it would pay dividends when reversing too.


Here is a handy bit of software which will help you select the right prop for your situation.

http://www.alberg30.org/maintenance/Mec ... opcalc.xls

Cheers,

Tom