102 props

[AusOB_Collector]

Hi all

Today I've been working on getting my other TD ready for our wet meet this coming Sunday. I have put the gearbox from AD13945 on it as it is in perfect working order, and with exams coming up every day this week I don't really have the time to get Te original gearbox to the TD working...

I know that this has been discussed on here before but I can't find the thread... Which prop (bow tie/hydrofan) should technically push the boat 'faster'?

I understand that Gulls weren't designed to go fast and they were meant as pushing power, but I'd like to hypothesise that one prop will push my dinghy slightly faster than the other, I'm not sure which one.
I will be taking both props to the meet, so let's place bets on which prop wins before Sunday then!!

Pics below are of the motor in question.


Cheers
BP

[Oyster 49]

Personally I think the "Swept" SD style prop goes well. No hard facts to support this though.

[Keith.P]

For looks, a swept back prop, but it will not fit a sub gearbox.
Bow tie props, I think they look ugly, but looking at your motor, it should have a hydro fan and they are supposed to be better anyway.

[headdownarseup]

I've sort of dabbled with this last year.
As a direct comparison using an early (ish) AC with the swept back prop and a later WSPC with a 5 bladed hydrofan prop, of the 2 the swept back prop pushed my heavy 17 footer noticeably quickly. Even my old bargepusher AHC went quicker than the silver century plus which was surprising to me.
HOWEVER, on your smaller "tinnie" i'd expect things to go either way.

Depending on how old your TD is, (cant remember without looking it up) i would think the correct prop for one of these motors would be a cloverleaf type. Not having much data on any of these yet doesn't help here so i'd play it safe and opt for the hydrofan prop.

Let us know how you get on

Jon

[Charles uk]

If I remember right your motor's lower unit 2.5 : 1 sub ejector as Keith so rightly stated, was only sold with or could use 4 or 5 props,
Bow Tie, 3 bladed clover leaf, 4 bladed clover leaf, & possibly 2 hydro fans, it's almost impossible to guess which prop would best suit your boat without knowing the all up weight & how draggy your boat is.

So the only way to get the correct answer is test, test & test!

This probably isn't the best site for a reasonable answer, it might be better to look at the Bermuda racing stuff on Facebook.

[AusAnzani]

Absolutely. The hull shape and weight will have a big impact on performance. Would be near on impossible to say which prop will work best.

In a different league, but when I set up my boat I tried 5 different props. The one ghat gave the best overall performance was the one least expected to do so. It (19" Quicksilver Laser II) is designed for light weight ski boats, very different from the 1500kg package it's currently pushing.

Regards
S

[Collector Inspector]

Interesting

Bring as many props as you have BP

My AC will be at the meet with the recon/pitched bow tie........you can use it on your motor as a compare thing aye.

I have a cut down 5 blade.......I have no idea what that will do.

I will bring a clover leaf as well.

You have GPS on your IPhone?

Cheers

BnC

[AusOB_Collector]

by Collector Inspector

Interesting
Bring as many props as you have BP
My AC will be at the meet with the recon/pitched bow tie........you can use it on your motor as a compare thing aye
I have a cut down 5 blade.......I have no idea what that will do.
I will bring a clover leaf as well.

You have GPS on your IPhone?

Cheers

BnC

Yes B I have a GPS fortunately, sounds like a plan!! Weather forecast is looking promising for the wet part of the meet!

Cheers
BP

[headdownarseup]

From a "garden shed mechanic" point of view, as you have a small (ish) light (ish) sort of boat to start with, coupled with a 102motor (in varying power outputs) as others have mentioned if outright speed is what's important , why not try polishing your hull first before you experiment with props. Sounds daft i know, but every little bit helps. Should be an easy job on your "tinnie".The less drag the boat has the more effort the engine can put into making extra revs into the prop.

A "re-worked" prop may end up with quite a noticeable difference in outright performance compared to a standard unfinished and dirty prop! I've noticed that an awful lot of these seagull props are quite a long way off perfection. Blades of unequal length, 1 or 2 blades of unequal weight, blades of unequal pitch all add up to something pretty nasty as a prop. Then there's cavitation where the prop loses it's grip on the water (a bit like the tyres on your car losing traction) None of these imperfections will help in the speed stakes.

However great or small those "improvements" are will be yours and yours alone. Ask a racer for the full in depth explanation in extracting the very last ounce of GO GO GO.
Polishing is easy, the other bits are more secretive and perhaps best explained in depth by someone that knows.

I personally wouldn't bother with a 5 bladed hydrofan as i'm not convinced a bone stock 102 has enough power in reserve to turn one of those.
I would stick with the more common props available e.g. the bowtie, 3 bladed "meat cleaver" as i call it,the 4 bladed hydrofan or if you can find one a proper 4 bladed cloverleaf type. All of them will vary in pitch and surface area, some will work better than others. Testing is the only way you'll find out which works the best on your little boat.
If you're going in "test mode" for your wet meet, why not try attaching a tacho meter of some description to your ht lead to see how many revs the motor is making. Record the results noting which prop was fitted at the time of test along with a speed measurement from your gps and this should give you some credible data to work from.
I'd be very interested to know how you get on with it as a fellow 102 enthusiast

Jon

[Charles uk]

I think you might find that sub ejector 102 props have 102 cast on the hub, well it's certainly on the bow tie, the 3 & 4 bladed clover leafs & the 4 bladed hydro fans.

Shark skin is considered one of the lowest drag finishes & that's not polished, Charles P as a yacht racer is probably the best to discuss low drag finishes.

[AusAnzani]

Polishing hull bottoms! Does it help to increase speed?

I guess in the “speed range” we're talking about here, polishing may have some impact, however in principal if you want to go faster you need to minimize surface tension and in turn the drag co-efficient. Polishing doesn't achieve that.

Think about it like this. If you run your finger along the surface of a smooth pane of glass, your finger will stick to it. By contrast if you run your finger along a sanded surface, it will slide along with much greater ease.

In “fast boat” applications, it is not uncommon to sand the rear third of the hull with a 320 grit paper. The rough surface allows air bubbles to form between the hull bottom and the water, which in turn minimize surface tension and drag. The result is higher speeds.

Have you ever wondered why planning hulls run faster in a small chop than they do on flat water? The simple answer is the more air you have under the hull, the lower the drag co-efficient will be.

Same sort of logic applies to many other items we take for granted. For example, the surface of modern day clothes irons are textured instead of being perfectly flat, golf balls have dimples. All is about minimising drag co-efficient.

It’s true your hull bottom needs to be void of any defects (tinnies are generally the worst), but I don’t believe that polishing will help to increase speed.

Regards,
S

[Hugz]

That's very informative S... nice analogies. Thanks.

[Collector Inspector]

I will bring a Mini-Tach

Yes, it will be a very "WET" meet indeed!

BnC

[headdownarseup]

What i meant by "polishing" (particularly with aluminium as is the case with a "tinnie" boat) was more of a scratched up surface using steel wool leaving behind fine scratch marks, not mirror polished by any means, but more along the thinking of shark skin. Dings and dents are something else entirely which might take a bit more effort to sort out.

Last year when i painted my own boat i was in two minds to go with a super smooth finish (a tonne of work let me tell you as it's pretty rough looking underneath the paint) OR not to worry too much about the prep work and flatten back the worst bits and re-paint.
Needless to say i chose the easier option, which when coupled with a seagull gave me some quite interesting improvements over the previous outings using the exact same motors.
Dont get me wrong, it's not going to break any water speed records but i've noticed a slight reduction in fuel consumption, the boat being what it is tends to stay on course a bit easier now without constantly having to correct steering inputs as before. Whether this is due to the new paint or not giving a slicker overall finish i have no idea, and to be honest i'm not that bothered. It looks clean and tidy and presentable which is what bothers me the most.
I'm happy with it at any rate.


Jon

[Merry]

So are the results in?