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Propeller Change


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Hi Folks,

I,m in the process of changing the prop on Destiny and wondered if anybody could assist with my choice of prop.

The current prop, which i have removed and is in my garage waiting for me to clean it up is a Mercury Marine Mirage Plus Stainless Steel one.

Its a 3 Bladed 15 1/4 x 17p which can't get the old girl on the plane.

I know i have posted before about this, but now i need to get this sorted whilst she's out of the water.

# Max RPM that can currently be acheived is 3400 (Need to acheive 4400)

# Max speed acheived 13 MPH (GPS reading, not water speed)

# Gear ratio at present unknown (forgot to read it from the outdrive when i was last down there)

# Weight of boat - Still unknown (Too many conflicting weights advertised)

# Weight of engine & drive 250KG

I spoke to Steel Developments, (Clements Marine only supply props for shaft drives) spoke to a lady on the phone desk and gave as much info as i had, but all she wanted to know was, what the current revs and what type of drive it was.

She then told me that what i needed was a 3 Bladed 16 x 13p. Slightly bigger diameter, but 4'' down on the pitch. (alluminium prop, as i don't want to pay for a stainless one)

Now, it is very likely that this will do the job, but my concern was this...........

The lower the pitch, the more revs / acceleration / lower top speed

Due to the fact my engine is only 120hp, i need to find the balance between getting it on the plane easily, but not too easily where the top speed is sacrificed once its on the plane.

There was one thing i didn't mention and that was that i have play in the remote and am not getting full throttle, although its almost there, so i don't think this has a big affect on the problem.

They didn't seem to be too helpful with the top end result, but was 90% sure that this size prop would get it on the plane though.

So, i though that i would post on here to see what you have to say about this.

(approx 200rpm per pitch) another variable figure depending who you speak to.......Lol

Price for prop including vat & delivery is £126 (Any cheaper, let me know please) if this is the size i should go for?

Also if anybody knows somone who wants a Mirage Plus Stainless Steel prop including flow torque hub, let me know, although i won't part with until i have tried out the new prop. So, basically summer time.

Thanks - Grant

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That sounds about right, but it would coming from SD as what they don't know about props isn't worth knowing. If you get that prop and it is marginaly under you can always have it cupped slightly to drop a few revs off, with that size motor you're not going to rip any trees up anyway whatever the prop size

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Thanks for the quick reply David.

Yes, the engine is a bit on the small side, but if i can acheive between 22 - 25 Mph, then i would be happy with that.

To be honest, my only real choice regarding this will either be a 15 or a 13 pitch, but as you said, SD are the experts.

I think i will see what other thoughts people have on this and then bite the bullet.

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Hi Grant,

I can't really offer any practical advice as we have a BMW220 petrol engine in our Falcon which produces higher revs than yours at WOT as I'm sure you or the previous owner re-engined to diesel.

Our prop is a 17x13 and with our engine, she starts to plane at around 12 to 14 kts dependant on conditions :wave

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190 Hp Grant cheers

You and David have really got me going now and I'm trying to find out what the WOT is for our engine (hope that makes sense) I know it's high and I reckon the only way I can find out for sure is to ring PH Marine (the UK BM Marine people) and ask again

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I have a feeling that you are going to struggle with that sort of HP, you may be able to get her on the plane and up to max revs but in order to do so there is not going to be a lot left for any more speed.

try a PM to Jonathan (mariotech) he had a Falcon that had been re engined with a small diesel, think he had to run a prop that had to much pitch and put people on the bow till it popped up.

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I've read through this thread carefully ( :) ), so I'll try to not duplicate anyone's suggestions so far.

When you took her out of the water, how much growth was on the hull ?

You mentioned that the control cables "couldn't quite" pull the throttle lever completely open ? It may be a red herring, but I would want to check that out first, (but difficult now, with the boat out of the water, admittedly).

Coming down from 17" pitch to 13" in one go seems a very big jump, given that she achieves 13mph and all but the last 1000 revs.

I hear what everyone is saying about Steel Developments, but as "she" only asked the revs and the type of drive (not the boat), it sounds very much like she has just applied the rather simplistic "300rpm per inch of pitch" rule.

As you say, you just need a little more prop torque to climb up the hump and you'll be away, then the 13" pitch might be too under geared and not give much more and will waste revs and fuel at all speeds, planing and displacement.

I appreciate the gamble (assuming, if it was still wrong, you'd have to sell it 2nd hand), but I myself would be inclined to try a 15" pitch first. At least if you still had to go for the 13" afterwards, you wouldn't need another lift, (assuming you could change the prop again with the leg tilted).

I would guess that the Falcon 22 is around a ton and a half, so 120hp shouldn't really be borderline for planing (IMHO :) ). I used a Hardy Pilot once that weighed just over a ton and planed easily with a 75hp. :)

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Hi Strowager,

Regarding the 15p, yes this is what i need to be sure of. I realistically have two choices here, either the 15 or the 13.

The weight of the boat is going to be somewhere around the 2300kg mark, as the Bayliner 2255 is 1800kg and they are very light for their size. (Hence the nickname everybody knows them as)

I have seen lots of varying weights for these boats advertised (obviously engine weight being a factor) but they all seem to fall into the 2000 - 2500kg area.

Also, bear in mind, she nearly there with the prop we have on there now as you can see in the photo

But keep the messages coming, because im having a good read of them and digesting it all.

PS - Don't say anything about Lifejackets - I KNOW LOLpost-810-136713777549_thumb.jpg

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Oh, and i think that was with 3/4 tank of fuel, a full fresh water tank an extra battery at the back and loads of clutter as we were on holiday.

Although, i would have been a little bit heavier on the way back after eating at all the pubs during the week.

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As you say Grant, the photo shows she's very nearly up.

I would estimate the LWL of the Falcon 22 to be around 17ft, assuming the 22 includes the swim platform as well as the bow overhang.

Your theoretical maximum displacement speed for that LWL is therefore between 6.16 and 7.11 mph.

( √LWL (in feet) x between 1.3 and 1.5 = speed in knots x 1.15 to get mph )

Above that, and she starts to climb the "hump".

So if you're as high as 13mph with that short length of hull then you're very nearly "up".

No guarantees ( :) ), but I know I'd try a 15" pitch first....

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Would you expect to make WOT before she pops up ? I think I would adjust the throttle linkage, service the engine and add some good diesel additive. Perhaps remove and refit the prop well greased whilst it's out the water so a prop change in the water would be do-able if needed.

Jonathan

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The 15 pitch prop does seem appealing........ but costly if its not quite enough.

It would be nice if there was a try before you buy shop. Lol

Changing the prop in the water is possible if i could get enough people to sit on the front of the boat, as even with the drive fully up, the prop is still underwater. Also i would have to be extra careful not to drop anything......

I have also contacted someone today by E-mail who might be able to help, so i will see what he has to say on this.

I would like to buy the prop at 17.5% instead of 20% vat, as i don't really like giving the government money for nothing, so i need to purchase it this side of Christmas.

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Hi Strowager,

Regarding the 15p, yes this is what i need to be sure of. I realistically have two choices here, either the 15 or the 13.

The weight of the boat is going to be somewhere around the 2300kg mark, as the Bayliner 2255 is 1800kg and they are very light for their size. (Hence the nickname everybody knows them as)

I have seen lots of varying weights for these boats advertised (obviously engine weight being a factor) but they all seem to fall into the 2000 - 2500kg area.

Also, bear in mind, she nearly there with the prop we have on there now as you can see in the photo

But keep the messages coming, because im having a good read of them and digesting it all.

PS - Don't say anything about Lifejackets - I KNOW LOL[attachment=0]Destiny action shot Original.jpg[/attachment]

Hi Grant,

I emailed the designer Andrew Wolstenholme and here is his reply regarding weight:-

Jim,

I'm afraid that I have no accurate weight figures for the 22. The hull

was designed to 2tons displacement floating to datum waterline but this

cannot be taken as an accurate indication of the actual build weight.

Regards,

Andrew Wolstenholme

I'd say your estimate of 2300 is about right and having spun ours on the "ropes" a few times over the years, I'd reckon 2500 (as SWMBO has filled her with many bits and bobs lol)

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Hi Jim,

Thanks for finding that out. TOP Falcon MAN

I have also since, mailed your friend, who got back to me very quickly and was very helpful. So thanks for that as well.

I also had a chat with SD again, and have now ordered a new prop.

This will be a 15 x 14 (so we will see happens when i try it next year) Fingers Crossed

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