What speed does your boat motor forward at lowest rpm?

F

fredrussell

Well-Known Member
Joined
24 Mar 2015
Messages
4,162
Visit site
Just out of interest really. My 31ft boat with 27hp engine will do nothing less than 2 knots in forward gear, in slack water. I reckon 1 knot would be far better for gingerly creeping into a marina berth. I’m just curious as to what speeds you lot get ahead dead slow?
 
Our Island Packet SP Cruiser with 110HP Yanmar turbo and 21X17 prop does 3.5 kts at idle in still water. It is often far too fast for ferry gliding. We need a good flow to slow forward motion. First Mate has asked for a slower idle, but then we get stalling. The idle speed is set for the best compromise.
 
I’ve never measured it, probably about the same, two knots. My normal method of approach is to take the boat close to the berth allowing it to slow almost to a stop, usually in neutral, and then engage gear if necessary to give me steerage way by getting flow over the rudder from the saildrive, changing to astern to bring the boat to a halt before contact is made, all depending on the geometry of the berth.
 
My last boat was a lightish 26 footer with an 8hp outboard. That would do just over a knot creeping forward - which was very useful for picking up the swinging mooring single handed as the tide in the Orwell seldom seems to flow at much more than that
 
fredrussell said:
Just out of interest really. My 31ft boat with 27hp engine will do nothing less than 2 knots in forward gear, in slack water. I reckon 1 knot would be far better for gingerly creeping into a marina berth. I’m just curious as to what speeds you lot get ahead dead slow?
Click to expand...
What RPM does it take to do 4knots? 5? 6?

My boat is in neutral more time than it's in ahead, when entering the marina.
I think it's over propped, but I'm not convinced the surveyor read the pitch correctly..

My simplistic understanding is something like this:
The slower a boat goes in flat water, the less force needed for a constant speed.
So the prop 'slips' less.
Going faster the prop is pushing water back as well as pushing the boat forwards, to do 50% boat speed you need more than 50% more prop RPM.
So a boat which does 6knots at 4000rpm might do 5knots at 3000 and 4knots at 2000 and 2 knots at idle?
Slip depends on the blade area of the prop, it's not all about the pitch.
It's not like the speedo and the rev counter in a car.

I'm still re-learning to park a boat, but my technique is mostly coasting through the marina with a squirt of throttle and helm for each turn.
 
Irish Rover said:
Neutral is your friend.
Click to expand...
Yes, but remembering that a rudder doesn't work as well as either in gear or at higher speed. Shoving the boat into ahead as start your turn then back to neutral as the boat turns will spin the boat without too much gain of speed, and if you do find you're going to fast, a quick burst of astern will slow you without throwing you off course too much.

Health warning: this applies to most boats to a greater or lesser extent. Try it somewhere where it doesn't matter first!
 
Stemar said:
Yes, but remembering that a rudder doesn't work as well as either in gear or at higher speed. Shoving the boat into ahead as start your turn then back to neutral as the boat turns will spin the boat without too much gain of speed, and if you do find you're going to fast, a quick burst of astern will slow you without throwing you off course too much.

Health warning: this applies to most boats to a greater or lesser extent. Try it somewhere where it doesn't matter first!
Click to expand...
Every boat I've ever driven has been different but, without exception, the slower you can do things the better and neutral has always been my friend.
 
Well, seems like mine is fairly normal then. Mine is a very easy boat to handle so it’s of no concern really, especially as my berth is wonderfully easy to get in and out of.
 
Stemar said:
Yes, but remembering that a rudder doesn't work as well as either in gear or at higher speed. Shoving the boat into ahead as start your turn then back to neutral as the boat turns will spin the boat without too much gain of speed, and if you do find you're going to fast, a quick burst of astern will slow you without throwing you off course too much.

Health warning: this applies to most boats to a greater or lesser extent. Try it somewhere where it doesn't matter first!
Click to expand...
More or less what I said, but the type of rudder and drive make a difference. A burst of throttle with my saildrive will not initially get the boat turning, even with full rudder, whereas on our old Sadler 29 it would throw the bow round. The small amount of prop walk can also be used on my 34, making turns ahead to starboard tighter than to port.

I had a guest boat-owner on board some years ago and he made a comment about something I hadn't even thought about. He had noticed that I often give a short burst of astern when moving ahead into a tight space to tighten the turn, which seems instinctive to me and can almost turn the boat in its own length.
 
Wing Mark said:
My simplistic understanding is something like this:
The slower a boat goes in flat water, the less force needed for a constant speed.
So the prop 'slips' less.
Going faster the prop is pushing water back as well as pushing the boat forwards, to do 50% boat speed you need more than 50% more prop RPM.
So a boat which does 6knots at 4000rpm might do 5knots at 3000 and 4knots at 2000 and 2 knots at idle?
Slip depends on the blade area of the prop, it's not all about the pitch.
Click to expand...
There are a number of things to pick apart here:
A fixed, non variable pitching prop is designed to work effectively in a surprisingly narrow range of RPM and speed, usually around 90% of max speed. 70% efficiency is about as good as it gets for a displacement prop.

A clean displacement hull has around 25-40% slip, but in a high speed planing hull this will trend towards 0%.

Total resistance is not a linear curve, but exponential. Consequently, the RPM or power needed to reach a certain speed is not a linear progression, i.e. : 1000 RPM per knot achieved. This explains why very little power is needed to move a boat at slow speeds, even though the prop will not be all that efficient or designed for such a low RPM/speed.

The ideal prop would be a large diametre, slow turning, single bladed one. For various practical and obvious reasons this is not or not always possible. A sailing boat's (fixed) prop will naturally be a compromise to reduce resistance under sail.
 
Wing Mark said:
What RPM does it take to do 4knots? 5? 6?
So a boat which does 6knots at 4000rpm might do 5knots at 3000 and 4knots at 2000 and 2 knots at idle?
Click to expand...

Depends on the engine and the boat.

I have just been doing the sums for a new engine for my Golden Hind. 31'6, 28' on the waterline and displacement 5.6 tonnes. Calculated hull speed 7.09 knots. Most engines in the frame are 29hp at 3600rpm (although one is 3200). Required cruising speed is 6 knots which needs 14hp, so approx 50%. In the 3600rpm engines this is typically 2400rpm and the 3200 is 2200. Maximum speed is 7.34 knots - so a 25% increase in speed needs double the horsepower. I also did the sums for the engine that is in currently which is 34hp at 2800rpm. Still needs 14hp which is at 1300rpm for 6 knots and a maximum of 7.79 knots.

The figures for the 29hp engine are pretty much the same as those achieved in my last 2 Bavarias which both had similar displacement, but slightly longer waterline length, and both with 29hp engines. The big difference is that the Bavarias achieved this with simple 2 bladed 16" props because the saildrive operates in relatively clean water whereas the GH has a long keel and the prop in an aperture so the challenge is getting as large a prop as possible in the space.
 
Stemar said:
Yes, but remembering that a rudder doesn't work as well as either in gear or at higher speed. Shoving the boat into ahead as start your turn then back to neutral as the boat turns will spin the boat without too much gain of speed, and if you do find you're going to fast, a quick burst of astern will slow you without throwing you off course too much.

Health warning: this applies to most boats to a greater or lesser extent. Try it somewhere where it doesn't matter first!
Click to expand...
Good tip and is how I do things getting into tight marina berths with our recalcitrant long keeler. Most useful lesson to come out of my coastal skipper course

However if you are trying to stem a contrary the current, going astern loses steerage fairly quickly and you can readily get blown out of line. I dont miss trying to pickup our old mooring as I have commented elsewhere
 
You must log in or register to reply here.

Share this page

Top