Cruise Speed Dandy 19 ?

[RS281168]

hello,
can anyone tell me what the maximum hull speed is of a DC Dandy 19??
The first engine was a 9.9 hp Yamaha Four Stoke and the speed was at about 10 miles per hour
but who was constantly at full throttle to come along.
Now we have a 30 hp Two Stroke and cruising more enjoyable,
the rate has gone ahead with a 4 to 5 km,
I have received information from the British Marine Federation that the maximum power output is to 75 hp.
I do not want to make speedboat but like it better when I have a bit more power  

[Guest]

All boats travel on water in two distinct different ways, and the laws of physics govern the maximum speeds by standard formulas.

"Displacement" speed is where the hull pushes through the surface of the water, and "planing" speed is where the boat lifts itself up and skims across the surface.

If the hull design can plane, and there is sufficient power to overcome the boat's weight and lift it onto the surface, then maximum speed is only limited by the power available.

If the hull design cannot produce lift ("displacement" shaped), or there is insufficient power to lift a planing hull, then the boat is constrained to "hull speed", which is a straight calculation from it's waterline length.

I would say that 30hp may be insufficient to plane your 19ft dandy, even though it is a planing hull.

You'll therefore be restricted to 1.4 x the square root of your waterline length in feet, which will be about 1.4 x 4.2 = 5.88 knots, which is 6.7 mph.

Increasing the power past that speed will only result in a huge wash and a very bows up trim to the boat, and waste a lot of fuel, unless there's enough to plane.

If you had the 75hp, or perhaps a 50hp, then it would easily break through this "hump" speed, and lift up onto the plane, where you could actually then throttle back a little, to achieve a comfortable cruising speed of about 20 to 25mph. (Only on Breydon on the Broads though !)

[JawsOrca]

Hi there, Attached is a picture of our first boat. This is her wizzing around, shes a CB 19 which although different from a Dandy and I guess a bit lighter she easily got on the plane with a 30hp 2 stroke.  But, and I was hoping Strowager would go onto explaining why you wouldn't want this! and in fact you should keep with a smaller engine!

 

Basically (and correct me if I'm wrong strowager) to get onto the plan takes a big engine so if you want to break the 6.7kts speed limit (which you 9.9hp engine should comfortably hit hour after hour) then you probably want a ?? hp engine. But if you don't want to get above this speed then there's no need to waste the money on buying a bigger engine and even back at low speeds (5 to 6kts) you will be wasting money paying for that extra engine (Kinda). To get on the plane and stay there at about 15kts (which would feel comfortable enough) I'd suspect you will need a 40hp (2 stroke or maybe 50hp 4stroke as 4strokes are heavier).. If you are happy at 6kts then stay with 30hp or even get a smaller 4stroke (we settled with a 18hp 4stroke in the end and that was nice on the rivers enough to get close the plane if wanted and cheap enough for river use). Some people have 2 or three engines and swop over depending on what use you here as we are an esturary so for sea use they put on a chunky engine and put on the 9.9 for river use... 

 

If you look through my post about my sabre project you will see more on this where strowager and I discussed the engine for that boat, and agreed that a smaller 30hp is fine.

 

Enjoy

[Guest]

....I was hoping Strowager would go onto explaining why you wouldn't want this! and in fact you should keep with a smaller engine!

 

Basically (and correct me if I'm wrong strowager) to get onto the plan takes a big engine so if you want to break the 6.7kts speed limit (which you 9.9hp engine should comfortably hit hour after hour) then you probably want a ?? hp engine. But if you don't want to get above this speed then there's no need to waste the money on buying a bigger engine and even back at low speeds (5 to 6kts) you will be wasting money paying for that extra engine (Kinda). .....

 

I didn't want to waffle too much on the subject, (and failed ).

 

You're quite right.

 

It's really a case of Sod's law with boats, the "comfortable" top speed most people would logically like their small boat to do is about 10 to 15 mph, and yet that's virtually impossible to achieve with boats up to about 50ft long because it's right in the middle of the "transition" phase between displacement speed and planing speed.  Even boats designed to go much faster will create a really big wash at those speeds and waste a great deal of fuel.

 

I was very frustrated by this when I bought my first small cruisers and re-engined them to go faster (at the coast).

 

I once had a 20ft planing hull cruiser with a 15hp outboard, which was quite happy at up to about 6mph, and could max out at about 8mph.  At a cost of over £5000, I re-engined it with a brand new 50hp four stroke outboard. It was devastating to find that it could then still only do a max of about 12mph, with a massive wash, and the comfortable low wash cruising speed was still 6mph.  I checked with other owners and found I really needed a minimum of 70hp for that particular boat.

 

It's all to do with waterline length, because at displacment speeds, boats try to "climb up" onto their own bow wave, so the longer the boat, the easier it is to push along.  A typical BMC 1500cc diesel will push a 40ft cruiser 2 or 3mph faster than it would in a 20ft cruiser, and use less fuel, because it's twice as long...

[LondonRascal]

Interesting discussion this – so I might add something more and pick the brains of the wise.

 

Does adding an extra blade say 3 to 4 bladed prop have any advantage in speed the boat can reach?

 

I understand if you increase the pitch you may ‘over prop’ an engine and result in it labouring/smoking I guess too if you had a larger prop the same may apply.

[JawsOrca]

HI Robin,

 

From the small knowledge I have (And I'm just bored at work. I'm really no expert as this stuff is soo complex!). I understand that the type of prop is determined by a few things; Boat length, underwater profile, Space available to fit the prop (between the hull and the skeg), engine/gearbox outbox. And not speed.

 

For example my old sailing 24ft boat had a Bukh 20hp but she was a full displacement, we only had 15.5 inches clearance. Ideally that engine wanted a 18inch 2 blade prop with a small pitch (to work nicely) however as we only had 14inches (you need half inch other wise it would cavitate on the hull which means the water pressure would eat the fibreglass!) so we went with a 3 blade.. the prop manufacture (cost me £400!) said we need a 14 x 14 inch (so 14 inch in diameter and 14inch pitch) he wasn't sure if a we want a 14 x 13 as he thought that would be too small.. and just mix the water up.. the 14 x 14 was perfect and she pushes along at 6 kts through the 1200 rev range... (tnnorris  

 

A mate of mine was trying to be smart.. thought he knew what he was doing and went from an 22 3 blade to a 24 4 blade and blew then engine up because the weight of his prop was too great for the engine output and blew the engine up! 

 

So as I understand.. you can increase the number of blades to decrease the size but the pitch is effected.  but don't go too mad otherwise your engine can't throw it (And blow up or just not hit correct rpms)! A too small or a really bad pitch means your engine just mixes the water up for the fishes! So its a careful and clever art form!

 

(I think outboards on racing boats are different because a small change to the pitch can give a couple more kts and that extra bit can help you race - but for us even at 15kts its not worth it).

 

(When are back in norfolk? I've watched all your videos! - Can you get a marthams one too!).

[ranworthbreeze]

Hi Alan,

Your first boat was very similar in design to the 17 foot 6 inch Woodwitch (a plan you buy to build the marine ply boat)

Far to many years ago we made one of these in a mates garage, I enjoyed doing the woodwork on this and using the bronze gripfast nails.

 

You had to make the frames and then make the keel (or hog I think they called it in the plans) the frames were softwood and the keel and various stakes were mahogany. 

 

Sorry for going off of topic.

 

Regards

Alan

[JawsOrca]

Hi Alan,

 

We did a lot of research on the boat with the help of PBO (many years ago) but that name doesn't ring a bell though. She was fibreglass but it did look like the hull was moulded from a ply boat though so they may have copied that design She was a good little boat and we sold her too quickly which I still regret today. Onwards and upwards though

[Guest]

Interesting discussion this – so I might add something more and pick the brains of the wise.

 

Does adding an extra blade say 3 to 4 bladed prop have any advantage in speed the boat can reach?

 

I understand if you increase the pitch you may ‘over prop’ an engine and result in it labouring/smoking I guess too if you had a larger prop the same may apply.

 

Propeller pitch, diameter, and number of blades is a very complex calculation involving  many factors including the ones JawsOrca has said.

 

The goal is to get the maximum thrust per revolution from the torque available from the engine.

 

The trouble is, boats only have a single forward gear, and almost all have a fixed pitch propeller, so that has to be a compromise, and can't be ideal for maximum revs and slow or cruising revs.

 

The hydrodynamics of the hull have a big effect too, with chines, skegs, keels etc. all affecting the prop's transmission of power.

 

I had a very small Shetland cruiser once, that had a 40hp outboard. It should have planed easily, but it got stuck on the hump at about 12mph, and the engine could rev no higher than about 3000rpm.

 

It turned out that the 12" diameter prop had a 17" pitch, which was standard for that motor, which was usually fitted to lightweight open speedboats.

 

I changed it for a 13" pitch, and it made a big difference, it could then rev to the correct 4,500 rpm, and it got up on the plane easily, reaching just over 20 mph.

 

Four bladed props are usually referred to as "high thrust", and when fitted to small cruisers they can "grip" the water better making slow speed handling more positive, especially reverse.