Sea Hornet 2015

[Dave MilbournParticipant @davemilbourn48782]

Look at the next built and I can change the prop angle by 5% would this improve the boat???

Ooo, Harry! This is dangerous territory! For a vee-shaped planing hull I maintain that it makes very little difference and I think that both Gareth Jones and the maths would back me up. HOWEVER there is a Very Big Beast about this Forum for whom it is a crusade – almost an obsession – to achieve a zero degree prop angle. He and I have agreed to differ (aka reached an uneasy peace/impasse/stand-off), but a shallower angle does make the shaft a lot longer unless you adopt a belt-and-pulley or geared drive system between motor and prop shaft. Frankly a slight change of prop would have more of a noticeable effect. (Heads down for incoming….)

I'll give it less than two minutes into the first sail before you push the throttle stick up through the top of the box "just to see how fast it goes…"!

Dave M

[Paul TParticipant @pault84577]

Hello Harry

As you might have gathered from postings on other threads I am the aforementioned Very Big Beast. Thankfully some of the forum members do have a modicum of education and actually know what they are talking about.

Reducing the angle of attack will result in a more efficient propeller.

Dave

In Grammar School terms: Both the maths and common sense say that a shallow or zero angle propeller is more efficient than one on the end of a steeply angled shaft, or to put it another way a steeper angled shaft will require a larger diameter propeller to achieve the same amount of thrust as a smaller propeller at a shallow or zero angle.

In Primary School terms: A prop which is parallel to the line of desired motion is in its most efficient position (look up airscrew) and the greater the angle increases from zero the more energy is expended by the propeller in trying to lift the stern out of the water.

Using crayon to explain and in very simple terms Newtons third law states that every action has an equal and opposite reaction. (shotgun principal)

Whilst I am always open to pushing at the laws of causality I would need a little more to go on than 'what some bloke down the pub told me'

Paul

Usual disclaimers about degrees of education apply

[Dave MilbournParticipant @davemilbourn48782]

Hello Paul (for it is He – the very beast of which I forewarned you…)

I see it was a trap well laid although we've been here before (sigh…), so for Harry's entertainment…

I have no quarrel with the integrity of your maths or the purity of your argument BUT a 5° difference in prop angle is not going to have a significant difference to the performance of a model boat unless it's to increase an angle which is already at some critically steep value. I used to understand cosines and vectors etc but somehow I lost the enthusiasm for them along the way to here-and-now. Gareth is much better at them. If I wished to improve the performance of a given model hull then it would be a lot simpler to try a different prop, reballast a little or add trim-tabs than drastically alter the hull shape, hang the motor upside down or fiddle around with belts, pulleys and the like. I'm relatively ham-fisted and uneducated so I'm forced to keep it simple.

I make scale models so I have little choice in the matter – I have to stick within the parameters imposed by the design of the full-size boat. As a designer of freelance models you could contrive all manner of hull shapes to achieve this state of theoretical perfection, although don't forget that I've built one of your designs and it didn't get that close.

The debate thus seems really only about the degree of compromise that you are prepared to accept i.e. my Huntsman {12°} vs your Ellie/Sea Spray {5°}. My model seems to perform happily as it is without wanting to become airborne or nose over **LINK** so I'm not going to worry unduly about the shaft angle. Don't forget, too, that I'm also a noted cheapskate – those long Sea Spray shafts cost me sixteen quid each!!

I think that the impasse we had already reached is a satisfactory position, given what they say about irresistible forces and immovable objects. Newton's Third in action?

Harry
Don't lose any sleep over it, mate!

Dave M

BTW My boozing buddy Dave B has no theory whatsoever about prop shaft angles – or indeed about anything remotely connected with what he regards as my 'toys' – so we content ourselves "down the pub" by talking rugby, politics, old colleagues and other such nonsense. It passes the time pleasantly enough.

(Usual disclaimers about level of alcohol intake apply. )

Edited By Dave Milbourn on 04/11/2015 16:31:21

Edited By Dave Milbourn on 04/11/2015 16:36:23

[ashley needhamParticipant @ashleyneedham69188]

I will throw in my penn`orth and say that I agree with Paul that a Zero shaft angle is likely to be the most efficient but also agree with DM in that in a model it is likely to make very little difference.

There…that was helpful eh!!

Ashley

[Dave MilbournParticipant @davemilbourn48782]

Ashley – you don't disappoint. Is this diplomacy the result of wisdom gained from such a long retirement? What was that lovely Tom Lehrer line? Ah, yes!

"Soon we'll be out among the cold world's strife – soon we'll be sliding down the razor-blade of life…" (from Bright College Days)

(What most won't have spotted is that PT and I have no fundamental disagreement – we're just fundamentally disagreeable!)

DM

[Paul TParticipant @pault84577]

I see it was a trap well laid

The huge neon arrows pointing to the hole and flashing 'TRAP' did help.

For the record and for those who are new to the forum Dave and I often exchange salvos of good natured disagreement. There is no malice behind it as we maintain a solid friendship and often exchange emails regarding the price of certain alcoholic beverages.

Our on-going discussion about propeller angles has been rumbling on for a while and as Dave said we are now at a friendly impasse (irresistible forces and immovable objects)

Its all grist to our particular mill.

Paul

[Gareth JonesParticipant @garethjones79649]

Assuming the propeller shaft is horizontal and the thrust from the propeller is 100 Newtons, the force pushing the boat forwards through the water will be 100 Newtons.

Assuming the propeller shaft angle is 5 degrees and the thrust from the propeller is 100 Newtons, the horizontal component of that force will be 100 x Cosine 5 degrees. The cosine of 5 degrees is 0.996 therefore the force pushing the boat forwards through the water will be 99.6 Newtons. I suggest you would never be able to tell the difference.

This reminds me of an engineering joke. If a naked man and woman stand 5 feet apart and then walk towards each other, halving the distance between them with each step, a mathematician would say they would never meet. An engineer would say they would get close enough for all practical purposes.

Gareth

[Martin Field 1Participant @martinfield1]

Just out of interest, the shaft angle on an Albatross speedboat, all 12 feet of it, is very steep, for obvious reasons. The boat is short, the engine is in the back. Even with no clutch and gearbox, the shaft is steep, yet an Albatross with its basic Ford 10 sidevalve engine will reach 35 knots towing a skier. With a Coventry Climax FWE, closer to 50 knots (and terrifying to be in!) So maybe in theory it's best to have a zero angle, but of course it's not actually possible, even with a vee drive and a hull step. And the Albatross or, say, a Delta, does very nicely thankyou.

But then…an outboard seems to be an easy speed machine and that does have a zero prop angle. On th right hull, a 25 horse Crescent can give you 40-50 mph on a suitable hydro.

Cheers,

Martin

[Dave MilbournParticipant @davemilbourn48782]

On the right hull, a 25 horse Crescent can give you 40-50 mph on a suitable hydro.

Martin
True, m'duck, but how much of that 25HP is wasted in the bevel gears?

Gareth
Priceless – as ever!

DM

[harry smith 1Participant @harrysmith1]

One Line.

One Hook.

One Worm.

Who needs a fishing trawler I caught dozens of fish.

It was only a simple question?

Harry

[Diede van AbsParticipant @diedevanabs87670]

Hi Martin, Dave,

While a steep propshaft angle might work on a full-scale vessel, it just so might not on a scale model due to scaling reasons: the boat scales down, the prop scales down, but the water…. not so much. For a fast model to look right on the water, it often need to run way faster than scale speed. At that point the propshaft angle might start to become a problem as it pushes the back upwards too much – and the bows down.

Having said that – this is not a hard law. Your model might run just fine with the original steep shaft angle.

Dave – your Huntsman runs beautiful! no denying on that!

Edited By Diede van Abs on 06/11/2015 15:31:45

[Dave MilbournParticipant @davemilbourn48782]

Diede

Thank you kindly, sir! The new Huntress actually runs with her bow a little too high and could benefit from trim tabs to lift the stern. Maybe the propshaft angle isn't steep enough – but I'll bow to Alan Burnard for such a sweet design and promise not to take too many liberties with the scale model.

Dave M

[harry smith 1Participant @harrysmith1]

Hi Dave

Have ewe tried moving the batteries a bit more to the bow, again???

I moved the battery on the grandson's boat about 30mm and it's handling the rough water like an off shore racer.

I had it flying off waves with the prop only in the water!!!!

Harry

[harry smith 1Participant @harrysmith1]

Hi ALL

I caught a couple of Poms trying to STEAL my boat!!!!!!

I got a few pics of these Poms to post on the web site!!!

They put a brave fight but my crew did capture these villains!!!

[harry smith 1Participant @harrysmith1]

Hi Fellow Boats

The has arrived for sea trails, weather light showers with sun shine and morning high tide.

Checked the balance point on the boat 10 inches from the stern, a bit more than I like but today is the day.

Gum boots on, system check, all OK.

Slow speed start to test , run up to 1/3 throttle, very nice, a few runs up and down the canal.

Turned circle with the small increase in rudder area very good!!!!

Up the throttle to half the boat seating up nice with about 1/3 out of the water.

Up the throttle to 3/4 the sat up to about 1/2 out of the water.

Boat speed very quick!!!!!

Some old clown in the back ground shouts out "Any more in the tank, mate?"

So YES full throttle, very, very quick, but, chine walking!!!

In for a water leak viewing, all dry in engine and radio bays.

The motor a bit to hot(about 15 minute run) for my liking, so, I will change from 3 blade 32mm x 1.4 pitch to a 2 blade 30mm x 1.4 pitch prop.

Also add an engine air scoop on the engine hatch.

Over all I very pleased with my little SEA HORNET 2015!!

Old boats can go very fast!!!!

Also had a talk to a real boat builder with over 50 years In the business!!

His idea on the prop angle is the lower the better, all the prop blades are working at the design pitch and requires less power.

Harry

[harry smith 1Participant @harrysmith1]

Hi All

The 32mm x 1.4 pitch brass prop works a hell of lot better and with the added air scoop on the engine hatch all heating problems fixed!!!!

The camera boat (Fantome) which on GPS runs 31 mph, the little Sea Hornet runs 3/4 throttle at the same speed.

I have cut the rudder angle to half, she was a little too good a turning.

I flat span it at half throttle!!!

Harry

[harry smith 1Participant @harrysmith1]

Hi All

I added a bit of weight under the front seat which moved the balance point to 280mm from the stern.

The bow seats down better, but, it also stays down a bit to much in the turns!!

I may cut back the weight a bit at a time to get it right.

[Author]

Posts