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Eventide Prop Size

Posted: Wed March 26th, 2014, 9:32 pm
by Nigel Vaughan
Help! I am trying to determine the optimum prop size for my new build 25 ft Eventide. The drawings seem to indicate 10" diameter. However, my last Eventide had a 12" x 9" prop with a 14 HP engine. Beta Marine also recommend a 12" x 9" prop for their 14 HP engine with a heavy displacement hull. There is a prop size calculator on the web (Vicprop) and I plugged in some numbers there and it came up with 12" x 7". What do other Eventide owners have?

Thanks, Nigel

Re: Eventide Prop Size

Posted: Wed October 22nd, 2014, 9:55 am
by Eventide Owners Group
There is a link to a free prop calculator program on the 'builders and restorers page of the site. You download it to your machine and use in there, very simple but it works!

Not much point in looking at the size of props on other Eventides, unless you have the same engine, same power and revs, same gearbox reduction i.e. 2:1 reuction, (some have 3:1! and bigger slower turning props, more efficient but a bigger drag under sail!!)

Also you would need to know if the other Eventide reaches hull speed under motor and at what revs.

For instance, I have a 13 x 13 prop on an Eventide 26, that stretched 6" on the waterline. I have a Beta 17 inboard that produces approx 14hp at the prop at 3600 revs.

I have 'overpropped' my engine by increasing the pitch on my prop from the original 13 x 11 size to 13 x 13 to limit top revs. Job for a prop expert! My top revs are 3000 now and the speed under power 6 knots. about as fast as mine will go under power.

This 'overpropping' means the motor works harder, not a bad thing for a diesel, but also that at 2000 revs I get 5 knots, my cruising speed, and the motor is quieter. As a side benifit the astern is more positive too.

With props it is very important to ensure that there is 10% of the prop diameter clearance between the blade tips and the bottom of the hull, or there will be excessive prop noise because of the interference from water leaving the tips of the blades under power.

Not many Eventides can therefore swing a larger prop than 13 inches, unless the prop shaft has been extended aft further for a transom hung rudder. This is a 'swings and roundabouts' game, the larger the prop diameter the more efficient under power, and you really want 50% efficiency or better, however bigger props cause more drag when under sail....

If sailing efficiency is the thing you then have to forget the conventional 3 blade prop we have been talking about here and either go for a 2 blade prop, that will stop naturally behind the deadwood in less flow, or even better one of the ultra expensive automatic props that will chose the 'correct' pitch for whatever hull speed and power, alting for sea conditions and feathering totally when sailing, but we are taling big money for these!

If you go to any of the engine manufacturers, depending on what motor you have go to yours, and ask, they will use the same calculation program.

It takes into acount, or should do, keel configuration, hull weight and length on the waterline. Beta did the original calculation for my prop 20 years back and it was the same size as the calculations i had made elsewhere, 13 x 11, however i found the noise of the motor screaming away at top revs was too much, thus the 'over prpping' and the later 13 x 13 pitch. Many use this trick to make life quieter, but you can only get away with it if you motor has a few hp spare.

If you tried it with a 6hp motor you would end up with les speed and never able to increase it. I would have thought min hp to over prop on an Eventide would be 12hp.

Seeing s this thread is a month old now, bet you have already figured al this out and are happily powering round! If not hope it helps.

Let others know how you got on by posting details here though!


Re: Eventide Prop Size

Posted: Tue August 29th, 2017, 6:13 pm
by Nigel Vaughan
Your reply was very comprehensive, John.

In the event I went for a 12" x 9" prop, which was identical to what I had on my old Eventide, with a Beta 13.5 HP engine and a 2:1 reduction gearbox. On my old Eventide I got 6 knots at maximum throttle and an engine speed of 3000 RPM, in this one (same engine, same gearbox, same prop) I seem to get up to 3600 RPM (max engine speed) too readily and get only just over 5 knots with it screaming away. So, like you did, I am going to "over prop" a bit by changing to 12" x 10" (i.e. another 1" on the diameter) which will decrease the max engine speed and maybe give more bite at top throttle and a bit more speed. We'll see!

All the best - I don't write much on here but I do enjoy the site.


Re: Eventide Prop Size

Posted: Tue August 29th, 2017, 10:34 pm
by Eventide Owners Group
Hi Nigel.

I assume you mean you are going to change the pitch an inch. But if there is a way you can add an inch to the diameter and not foul anything and still have 10% of the diameter clearance from blade tip to hull even better, as the larger the prop the more efficient. (The racing boys have small props because of the additional drag of the larger more efficient props. But we are talking Eventides here and efficiency is a far better option!

I overpitched mine by 2 inches to 13 x 13 (on the pitch) and that appears to be about right for my 26ft plus boat and the Beta17 as the top revs are now 3000 and the speed unchanged at 6 knots flat out. Throttling back to 2000 gives a much quieter motor and still a good 5 knots in still water.

The astern is certainly better as it has more bite and as an aside the more pitch you apply the more the paddlewheel effect when coming astern.

I use this to great advantage as a burst of astern kicks my stern nicely to port, enabling me to reverse out of one of the tighter berths in Bradwell marina, a quick burst ahead on opposite lock and the prop thrust stops me going back to far, clouting boats opposite, it kicks the stern further to port, another quick burst astern and I am pointing the right way, enabling me to throttle back and steer stern first out through the pontoons, much to the amazement of other boat owners, some of whome have a real problem going astern! Many are either too timid or use too much with the wrong helm. Always helm over first!

From what I understood from the prop man who adjusted mine, 2 inches is about the max they can do and he reckoned it would reduce revs by 400 per inch extra pitch.. As I was reving up to 4000 before, motor screaming and more than the 3600 or 3800 it should have done, I think he was spot on.

Another thing I do every year is polish my prop till I can see my face in it. As an ex proffessional boatman I know a polished prop is far more efficient. And as an aside I never get any fouling on it! Some boats near me whose owners paint their props, are pulling balls of weed about by this time every year.. could never see the logic in painting a prop, especially as most used the same stuff they painted the boat with, a self eroding paint. Amazing how quickly it will erode at 1500 revs!

Hope you get some miles under keel, by sail or motor.

Owner builder
Eventide Fiddler's Green