World Sea Fishing Forums banner

1 - 20 of 28 Posts

·
In A World Of His Own
Joined
·
2,954 Posts
Discussion Starter · #1 ·
hi just seen a boat for sale a landor15..with a 6 hp outboard it say it does 6-8 knots my mate has 25hp on his and does the same speed is this correct can a 6hp make the boat go at the same speed as the 25hp outboard cheers andy
 
G

·
Different hull shapes will give different speeds.

A displacement hull will have a top speed of about 8 knots regardless of engine size. Your friend, if he owns a displacement hulled boat may well have far too big an engine on his boat and be simply burning fuel needlessly...
What boat does your friend have?

Tom
 

·
In A World Of His Own
Joined
·
2,954 Posts
Discussion Starter · #3 ·
hi he has a landor 17 its the same as the one for sale just slightly bigger
 

·
Registered
Joined
·
1,528 Posts
Then he has an engine that has more horse power than he needs, simple as that.
There is a mathematical formula that explains it all which is.
Max displacement speed of a none planing hull (Yours and his) is
The square root of the waterline length X1.5 = fastest displacement speed.

ie Water line length 16ft X square root = 4 X 1.5 = 6 knots.

It's phisics and can't be argued with or altered so if you get someone claiming 20 knots in a Landor he's fibbing or mistook :)0)

Afishionado
 

·
In A World Of His Own
Joined
·
2,954 Posts
Discussion Starter · #5 ·
hi i can see that from the maths ,, so in practice the boat he has will go just as fast with an engine half the size ,it does seem strange .. so does that mean a smaller uses all of its pushing capabilities and not waste it as i assume the bigger engine does cheers andy
 
G

·
hi i can see that from the maths ,, so in practice the boat he has will go just as fast with an engine half the size ,it does seem strange .. so does that mean a smaller uses all of its pushing capabilities and not waste it as i assume the bigger engine does cheers andy
Becasue of the shape of the boat it simply pushes water out of the way rather than climbs up and over the top.
Pushing water is quite hard work, but only to a point (6 or 8Hp worth of hard work). Once you reach the maximum speed of a displacement boat, no matter how hard you push it (all 25Hp?) all you will be doing is wasting energy and fuel.

In my opinion, if the ideal Hp is (eg) 8Hp a 10Hp would be the perfect choice as you could run the engine at less than full throttle to comfortably maintain top speed, but the engine isn't so big that is simply using a load of fuel.
 

·
In A World Of His Own
Joined
·
2,954 Posts
Discussion Starter · #8 ·
hi the boat came with the 25 hp and i have assumed it was matched to the boat incidently i have now secured my own mooring and was looking around for a boat to use for the summer when i spotted the other landor with the smaller outboard i plan to get a bigger better one next year but want a usable boat now to put on the mooring and as my mate has a pilot17 ready to go soon i was thinking that the landor would be fine for me in the interim cheers andy
 

·
Registered
Joined
·
52 Posts
If you want to know more about hull speed i suggest you read a little bit about the Froude number. William Froude was an engineer who made the first reliable laws for the resistance that water offers to a ship, wich leads to hull speed. The froude number is the multiplyer in the equation stated above.
 

·
Captain Chaos!
Joined
·
3,694 Posts
hi just seen a boat for sale a landor15..with a 6 hp outboard it say it does 6-8 knots my mate has 25hp on his and does the same speed is this correct can a 6hp make the boat go at the same speed as the 25hp outboard cheers andy

Hiya, I fell into the same trap...:)....I once bought a displacemet hull boat with a cracker of a Yamaha 30hp on it....it would only do around 8 knots flat out....any more throttle only increased the revs....I dont have it anymore, but it will soon have a much smaller engine on it, and it will do the same speed....:):)
 

·
Registered
Joined
·
1,528 Posts
The propeller pulls water from infront of it's self and pushes it out behind. Because every action has an equal and opposite reaction the thrust thus created is transfered to the boat and the boat moves forward..
As the boat moves forward a fresh chunk of water is presented to the prop' so it can chew it's way into that and chuck it out behind.........
A displacement boat has the resistance of the water infront of it to hold it back, unlike a planing hull which can slide over the top of the water......
So given that a displacement boat can only go so fast, if its engines propeler chews through more water than can be given to it by the boats forward motion it digs more water from BELOW thus creating a hole. The boats arse then sits down in the hole and the bow rises up to the top of the hole. The more power that is applied the hole gets bigger and the stern gets lower and the bow gets higher BUT the boat can not go any faster because it is now trying to climb a large hill that is constantly being created in front of it. In fact it might actualy go a bit slower. Worse than this it could sink if the power is suddenly shut off. The boat could slip back into the hole it has created and be swamped by the water rushing in to fill the hole. Don't rais your eyebrows in disbelief it has happened several times to ocean going tugs, and to a lesser degree of loss to small dinghys with big engines.

Afishionado
 

·
In A World Of His Own
Joined
·
2,954 Posts
Discussion Starter · #12 ·
hi blimey i was thinking of buying it but again im having second thoughts what size engine should if have....Andy
 
G

·
10Hp should be spot on.
This will give a couple of Hp over what is required, but mean that you can back the throttle down to a slightly more comfy RPM. This will help save fuel and prevent the need to thrash a titchy outboard.
Anything from about 6Hp to 15Hp will do the job, but below 8Hp and you will go everywhere flat out on the engine (noisy, bad for the engine and juicy) and much over 10Hp will be wasted.
 

·
In A World Of His Own
Joined
·
2,954 Posts
Discussion Starter · #14 ·
hi so running at half throttle on the 25hp will not be the same as running 3/4 to full on a 10hp..the boat does seem to go well , it uses about 6 litres for a round trip of 8miles.. is it possible then the boat is a semi displacement and not a displacement hull i have looked on some sites and the disciption is like a half of a lemon for a displacement hull the landor is not exactly like that i know im showing my ignorance but what afishinado says has been playing on my mind if i was to buy it and take the kids out it will be another think to worry about..so downsizing will be something that i would have to do for piece of mind even thou the boat seems to go and handle well have a mooring sorted now so its worry time until i have been and got one cheers andy
 
G

·
Hey Andy

It's a displacement hull.
AF's description is a possibility, but about as likely as winning the lottery. AF was describing the theory that in principal a displacement hull can dig such a big hole for itself in the water it simply falls in! Very slim chance.

That sort of fuel consumption is typical of a similar sized planning boat running at medium planning speeds (eg: 18 knots) with that engine.
In brief you would expect a bit over a mile per litre with that sized engine running to about 1 or 1.5 litres per mile for a nice big 140Hp on a planning hull.

On a displacement hull you should barely sniff fuel with a modern engine and still be pretty frugal on an older 2 stroke. Your current fuel consumption is very high, in my opinion, and on the right boat (maybe an Orkney Strikeliner) that engine will be giving twice the speed for the same amount of fuel.
 

·
In A World Of His Own
Joined
·
2,954 Posts
Discussion Starter · #16 ·
hi so i can now gather that the boat is a mis match ..i had thought about buy a strikeliner .. i would like to get a boat asap just to sit and secure the mooring i do plan to get a better one next year as i had planned but a boat like the landor would be fine just for now cheers andy
 
G

·
Andy

It may be a mismatch but it isn't the end of the world.

Assuming you are aware that the engine is way to big you simply have to get in the habit of running it at very low revs (find the optimum revs to obtain 6 to 8 knots). Assuming you do this and then don't try and rev the engine beyond that you will maintain relatively sensible fuel consumption (relatively is the word as it would be far better with the right engine) and you won't find you fall into your own hole in the water.
 

·
Registered
Joined
·
1,528 Posts
ANDY!!!! (afishinado says has been playing on my mind if i was to buy it )

You are over reacting BIG TIME. As the man on the insurance advert says ""Calm down dear"

You are looking at things in black and white terms when all are shades of grey.

Let us suppose you bought the 25hp outfit. There it is sitting in the water empty. WILL IT SINK? no! You put you and 2 people in it WILLI IT SINK? no. You put you and two people all on the edge at one side and get you to jump up and down. WILL IT SINK? Probably.
Was the sinking bit anything to do with the boat? NO. Was it due to stupid behaviour? Yes
Answer the boat is OK just do not do stupid things in it.

The 25 hp engine is fine, just more than is needed. If you want to be silly and open up the throttle full blast and then after 200 yards suddenly stop, the back wash will slop over the back and make your bum wet if you are sitting there.
So to make sure that does not happen WHAT DO YOU NOT DO? Answers on a £50 note please.

Afishionado

PS I am worried about your worrying. Like most things one learns by mistakes. You WILL make mistakes but mitigate them by common sense. If you are getting as nervous as you seem to be before you actually get a boat you are going to be a bag of nerves once afloat. No boat is perfect. No weather is perfect. No tidal current is perfect. No circumstance is perfect. All of them need to be combined into a best option at the time decision.
You seem to be trying to get perfection and total certanty before you even get out on the water. YOU WILL NEVER BE ABLE TO DO THIS!

Do you think boating is for you?
 

·
In A World Of His Own
Joined
·
2,954 Posts
Discussion Starter · #19 ·
hi afishinado wow you have summed me up better than my mum..i do tend to worry about things far more than i should .. but i have had some very bad luck in my life ( my father died after accident ) so i think its in the back of my mind that disaster is around the corner and now having been diagnosed with a seriuos illness i feel its not all fun and games...if you know what i mean but going back to this boating i do want to go ahead and as i have said before i will use it probebly mostly on my own is that a mistake or should it be a 2 person or more pursuit... i can see were you are coming from and do appreciate the comments and dangers you point out as i must admit on first experiencing boating these things you have pointed out do not seem obvious as its alright letting someone else do the donkey work but i now realise that its far from the calm and sereal hobby it first appers to be cheers andy
 

·
Registered
Joined
·
1,177 Posts
Best advice I can give you is talk to some of the guys where you moor the boat or if you have any mates with boating experience get them to come with you on the first few trips. You will pick up far more from being in the boat with them than reading forums or books.

Once you have got a bit of knowledge under your belt then you can go out alone but a) it can be boring and b) you need to be that bit more safety conscious.
 
1 - 20 of 28 Posts
Top