Poor Performance.


Senior member
Jun 30, 2004
Cruising area
Kings Road
Well due to my sons very poor health i had not had chance to put my boat through her paces until yesterday.

The boat is fitted with a 330hp mercruiser with Bravo leg and Mirage prop (19" pitch, i believe).

The boat handles superbly, slicing through the chop and tracking very well indeed. However, the performance is not what i would expect from this boat.

She seems slow to get out of the hole and only reads 4,200rpm at WOT. Guesstimated top speed 40ish. Didnt have my GPS with me and the boat speedo struggled to read beyond 25mph!

Any pointers as to what could be wrong/or what type of prop i should try.

PS. there also appears to be a light vibration running through the boat that could be prop related, however the prop had just been serviced after having all three blades bent on the bottom.

Well asuming a few details, such as drive ratio, accurate tachometer, and accurate speedo, and of course that it is indeed a 19" prop, any number of which could be wrong, 4200rpm with a 1.5:1 drive @ 15% slip =43mph.

Do you have a gps?

I don't know your boat model well, but understand they are quite heavy, even so, I would have thought low 50's should be feasable with that power.

Vibration could be the universal joints in the drive/gimble assy, this could also be soaking up a little power.

I don't want to try and teach anyone to suck eggs, but you are trimming the drive out when trying to achieve top speed aren't you? without doing this 43mph would be quite respectable!

Other than those points, it only really leaves two options, you're not making the full 330(tounge in cheek)hp, or the boat has a load of extra weight somewhere such as trapped water within the hull.
Drive ratio is 1.5:1
Tacho is believed to be correct. When i originally tested the boat they borrowed a prop which allowed the engine to exceed 5000rpm. (Dont know what type of prop was fitted though).
The Mirage fitted is definately a 19".

I do have GPS and will use it the next time i am able to take the boat out.

The manufacturers claimed approximate weight is 1822kg.

The leg was trimmed out when running WOT.

I believe that the Formula 242/330hp package should make around 55mph.

I guess what i would like to clarify is what prop i should be using in order to achieve the best top speed. As the boat came from windermere i dont know if it had its prop changed for waterskiing etc.

I would of thought that this rig should be using a 21" prop?

But im just a knucklehead and thats why im asking for the opinions of the boating gods.

PS. I think that this engine should be running at 4700rpm at WOT?
Last edited:
Just did the calcs, and yes, I think the WOT of a 330 is 4600-4800

@4700 with a 21" prop, asuming 15% slip,which is a reasonable assumption would give you 53 mph, so I reckon your bang on with ya 21"

trouble is, you're running a 19", so it should be revving it's wee nuts off! but it aint! so we're back to either down on power, or a load of extra weight you don't know about (like water trapped beneath the floor)


How do i tell if theres water trapped in the foam in the hull bottom? (The cabin and bilges are dry, and the surveyors report showed no moisture in the hull. The boat had been out of the water for three weeks prior to the inspection).

I am embarrassed to say that the waste water tank is full and i cant seem to empty it!!! any ideas?

Anyone got a nice 21" prop i could borrow?

That's not so easy. I don't know how your floors done, or what compartments there are to get flooded.

Where in the boat is the waste tank?

The tank appears to be in the floor just before the cabin door.


The Formulas do have solid hulls (not sandwich type construction) but the voids below the floor are foam filled to aid bouyancy.

I have a fuel filler style cap marked waste on my starboard side, but i thought that this was the water holding tank for the sink and chemical toilet. My toilet is a cartridge type.

I have a manual pump type tap for the sink. It dont matter how many times you pump it nothing comes out!!!! If you put your finger over the end of the tap it pressurises so the pump is working.

Any ideas chaps?
Ive just had a look on the yank forums and they suggest a stainless Bravo 1 four blade prop with 22" pitch.

Other owners of 242's have found these to be the best props giving a GPS speed of 55mph and good acceleration.

A cartridge johnnie is a bog that you empty over the side by hand!!!:drain:

I have no water tank filler on the bow!

No i dont think shes waterlogged either.

The engine revs freely in neutral but only makes 4200rpm in drive. However, she made over 5000rpm on test with a smaller prop. So i think that the prop is the problem.

I will double check on the installed prop tomorrow, so tune in for another exciting episode of "What's Your Prop".

Ask him what specifications his boat is and what his performance figures are please.

Oh and how you empty the waste water tank. :drain:
It would have to be a mighty shite prop to soak up enough power to reduce your revs to 4200 on a 19!...don't add up!

either it aint mekkin the power, or it's as evvy as ell (or a bit of both)
R-Don said:

Anyone got a nice 21" prop i could borrow?


Yep, I got a 21" Mirage you can borrow, I'm in Alderholt Nr Fordingbridge. Your welcome to come over and snaffle it!
Cheers TD,

Are you anywhere near Home Farm in Alderholt?

I am going to double check on my prop today if it stops raining and then ill get back to you.

Dunno, sounds familiar, but both me and wife don't know it. We're opposite tennis courts, and next door to riding stables!
Had a 242 ss called Habenero with a Volvo 330 7.4 litre and a Duo prop - managed 50 mph on a good day if that helps and I have no idea what it revved to!
Well i went down to double check on the prop and found it to be the following:

Quicksilver Mirage 14" with a 21" Pitch.

The label on the engine airfilter cover gives the motor a WOT setting of 4200 to 4600rpm.

So she is propped correctly but doesnt appear to be performing very well!!!

I will take her out with a GPS this weekend and see what results i get with that.

Does anyone know how the boats speedo works and if it can be cleaned up to work in a reasonable fashion.

Hoping to make the fireworks Friday, just depends how my sons operation goes this Wednesday.

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