Non Planing Hull - Outboard Size

Spannaz

Spannaz

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Medway Towns, Kent
www.kentwindowfilms.co.uk
Hi,
First post here so hopefully it's in the right place.

I have purchased a Starley Sea Nymph, 17ft boat with what is known as a 'non planing hull'
Now i kind of understand that this makes it stable, but not fast. However, i use this coastal waters and estuary's, and it's currently got a Mariner Magnum 10hp fitted to it. This pushes it along, but i would have to say it's not exactly moving at any real pace, and it HAS to be flat out when against the tide to make any headway.
Now my question is, would a larger 20hp increase the performance or does the hull restrict how fast it can go regardless of the engines output.

I would have thought that running an engine constantly flat out would be bad, but maybe this is normal?

Hopefully some kind soul can answer some of this for me?
thanks
Stuart
:)
 
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I would have thought 10hp was more than adequate for a 17ft boat.

A non planing displacement hull cannot be made to go any faster than its water line length allows. Probably that will be not much more than 5 knots for your boat.

My 19ft sailing boat has a 6 hp outboard for auxiliary power. That is quite adequate

An outboard should be happy to be run at close to its specified max rpm ... Usually of the order of 5000rpm but see the manual for your engine. However engine power and propeller size ( diameter and pitch) should match the boat. Ideally the boat should reach its maximum expected speed at a little under max rpm for the engine.

But are you sure its a non planing hull ? A totally different kettle of fish if it is a planing hull. 10 hp probably will not be big enough
 
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Hi, and many thanks for your reply. The boat in question is a Starley Sea Nymph, and my information is only based upon research from the net. It sits flat on the mud rather than at an angle, and from what i found it's because it has this type of hull.
I plan to get the engine off over winter at some point to get it serviced, and thus will get the more technical guys to ensure it is the right setup with prop etc.

My concerns were if it only does 5 knots, how would it cope in a fast current when the tide is turning. IE if i had a 8 knot tide, would i be going backwards flat out?
 
The control box and cables will do for another engine. However the fitting kit to convert the engine from tiller control to remote may cost up to £200 (Mercury for instance. Although the same kit for the Tohatsu is about £140. - It's the same kit, different bag)

A quick Google confirms that it is a displacement hull.


Picture_0604.jpg
 
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Lakesailor said:
The control box and cables will do for another engine. However the fitting kit to convert the engine from tiller control to remote may cost up to £200 (Mercury for instance. Although the same kit for the Tohatsu is about £140. - It's the same kit, different bag)
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thanks for that. I was hoping it would, so ideally if sourcing a 'used' engine get one that has already had the remote tiller control.
 
Lakesailor said:
You're going to be disappointed I think. It'll possibly get to 6 or 7 knots, but make a load of noise and use a load of fuel.
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is that purely down to the hull design i take it? So bigger engine will just make no difference other than like you say, alot of noise and fuel !


scrub that just looked it up, max speed is 6.8 knots based on 17ft waterline (learning by the minute here)
so basically ANYTHING abouve 6.82 knots is wasted, once i reach that speed i should just maintain it, as trying to push any more would be wasting fuel.
 
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My concerns were if it only does 5 knots, how would it cope in a fast current when the tide is turning. IE if i had a 8 knot tide, would i be going backwards flat out?
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You have to learn to work with the tides ! You wont be able to beat them.

Well done LS for finding the picture, I couldn't.
 
VicS said:
You have to learn to work with the tides ! You wont be able to beat them.
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perfect. That makes sense, however, if it's rough i won't be going out far anyway, prefer to learn to walk before i can run.
So, until I learn to read tide movements (times and heights are easy) stick to the safe channels.
Worries me that some people DON'T learn this before taking to the helm of a boat, and heading straight out to deep channels! :confused:
 
It is possible to exceed the hull speed with lots of extra power. But the boat will not climb onto it's own bow wave. It will just squat down in the trough and push a huge amount of water aside.

This is my 15ft boat (with a very similar hull to yours) at 6 knots. About 5.8 was it's hull speed and full throttle would get you to 6.3 knots (by GPS). But the noise and vibration just wasn't worth it.
You can see how it is sitting in the trough of it's wave.

That was a 5hp motor. 10hp should be more than ample.

20May6knots02.jpg
 
My displacement hull Viking 20 (1.5 tonnes) with a Honda 20hp on the back running flat out at all of 6 knot on the GPS :o. Note the big bow wave :eek:.

jgw78F9790s.jpg
 
Deefor said:
My displacement hull Viking 20 (1.5 tonnes) with a Honda 20hp on the back running flat out at all of 6 knot on the GPS :o. Note the big bow wave :eek:.

jgw78F9790s.jpg
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If you stowed the fenders you'd probably get another knot :D
 
Hull design

When talking power boats it is a question of ability to plane or not. Non planing PB tend to have a hull shape like a sail boat. it is designed for minimum drag in the water. This is most obvious at the stern where the hull lines will rise so that the transom is barely in the water when stationary.
A planing hull will have straight lines to the bottom of the transom which when stationary will sit quite deep in the water. This long flat area helps to lift the hull more easily onto the plane. So that the bottom of the transom is at the water surface. However the drag is much worse when not planing. Of course there is much more to hull design than this but it makes it easy for a novice to see what the hull is meant to do.
In a non planing situation as said the boat tries to make bigger bow and stern waves as "hull speed" is approached. Hull speed in knots is often given as square root of water line length in feet.
However hull speed is just a point on a graph of horsepower and resultant speed. At low speeds a small increase in HP will give a big increase in speed. ie between 1 and 2 knots. However as you approach hull speed the increase in speed with increase in HP becomes less. So that on your boat to increase from 10HP to 20 HP might increase speed from 6 to 7 knots. if you went to 50HP it might increase by another knot. Double the horsepower again might only give .5 knot increase.
Ultimately I guess any hull can be made to plane just as any stone can be made to skim the water. It is just not practical with your hull type compared to one designed to plane where the drag at about 6 knots is huge until the hull lifts itself above the water. Then there is often a noticeable increase in speed and certainly any increase in HP will give a lot more speed. This does depend on weight however.

We are talking water speed of course. Tidal movement may mean that you still actually go backwards if the world you are floating in is moving in the wrong direction at great speed.
good luck and enjoy the tranquility and cheap running of a slow boat. olewill
 
thank you for all your responses, certainly has given me a huge insight into hull designs, and what is and is not suitable.
Clearly i have stable hull that will plod along all day at 6 knots, use a small amount of fuel and be reliable if maintained well, what more could i want?
I have 3 young boys (8,5 and 3) and to be honest deep sea fishing is not something i want to do them aboard. But, the idea of the boat is that I can do things with my boys, rather than wave them off on the shore line. Some people may find boating an escape from family life, i tend to look at it as quality time.

What i will do is learn, learn and learn, listen to advice from others with more experience, and if in doubt ask.
Happy sailing :)
 
See you're in the Medway. Did you get the boat from Roger? The pic looks like the one he was selling.
Tides - 8knot tides are very rare around our coasts. 2 to 3 knots more like in the Medway. It's still crucial to work tides though.
 
Spuddy said:
See you're in the Medway. Did you get the boat from Roger? The pic looks like the one he was selling.
Tides - 8knot tides are very rare around our coasts. 2 to 3 knots more like in the Medway. It's still crucial to work tides though.
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hi, no i bought this through a friend, who only purchased it at the early part of the year. He has moved on to a bigger boat to do charters on.
It came to him from a guy called Kevin Jarvis on the Isle of Sheppey.

It's now moored up at the Kingsferry Bridge, and i live on the other peninsula near Allhallows.

Thanks for the info re tides................yes i have a lot to learn about tides!
:)
 
Spannaz said:
hi, no i bought this through a friend, who only purchased it at the early part of the year. He has moved on to a bigger boat to do charters on.
It came to him from a guy called Kevin Jarvis on the Isle of Sheppey.

It's now moored up at the Kingsferry Bridge, and i live on the other peninsula near Allhallows.

Thanks for the info re tides................yes i have a lot to learn about tides!
:)
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Boating is great innit! :)
 
I suggest you buy the chart for your preferred boating area - it should mark any particularly strong tidal currents.

Normal along-shore tidal currents shouldn't exceed a couple of knots, but may be much more where the tide rounds a headland. Esturies, particularly those with a relatively narrow mouth, can have very fast tidal current at certain states of the tide (particularly, I expect, falling tide halfway between high and low).
 
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