Advice from those in the know

[LadyPatricia]

It appears that our little Norman 24 is probably going to need a new engine.

ive been in contact with a company that will supply an engine and an adaptor plate to fit to our saildrive. My question is will the 2 cylinder MP-214 be man enough for our boat, it's a 14hp, or should we go for the 3 cylinder MP-321 which is a 21hp unit?

http://www.marine-power.net/micro-14hp---21hp.html

any advice from those in the know will be much appreciated 

 

Steve & Deb

[BroadScot]

33 minutes ago, LadyPatricia said:

It appears that our little Norman 24 is probably going to need a new engine.

ive been in contact with a company that will supply an engine and an adaptor plate to fit to our saildrive. My question is will the 2 cylinder MP-214 be man enough for our boat, it's a 14hp, or should we go for the 3 cylinder MP-321 which is a 21hp unit?

http://www.marine-power.net/micro-14hp---21hp.html

any advice from those in the know will be much appreciated 

 

Steve & Deb

Hi Steve,

So sorry to read that you require a new engine. I know nowt about marine engines, but would think because of tidal flows the bigger engine maybe the one to go for.

I am sure one of the many clever clogs on here will be along to give you good sound advice on which to go for.

Iain.

[JennyMorgan]

Just for the record I use a 4 hp sail-drive on a 21 foot boat, even going to sea with it. At less than full wellie I can reach max hull speed with ease. Size isn't everything! I really would have thought 10hp would be more than enough for Broads use. In other words, in reply to your question, I would be content with 14hp option.

[LadyPatricia]

Thanks JM.

do you not have to take into account the type of hull though? I believe Lady Patricia has a semi displacement hull, I stand to be corrected on that.

i seem to remember reading somewhere that for a sail boat it's worked out at 5hp per tonne, a displacement hull is 10hp per tonne and so on, again I stand to be corrected.

 

Steve

[Guest]

Sorry to appear to be just wanting to contradict Peter, but there are other important factors to consider.

A believe the Norman 24 has a planing or semi displacement hull, so it won't slip through the water as easily as a true displacement craft, especially a sailing craft, which maybe Peter's example is.

14hp is right on the lower limit of outboards used on 24ft motor cruisers on the Broads, and some people find them a little underpowered against the tide or on Breydon.

It might be safer to go for the 21hp, since it might be much more expensive to up-engine a diesel inboard afterwards, unlike with outboards.

[ranworthbreeze]

Hello Steve & Deb,

I see from your link that both engines only achieve full HP at 3000 rpm. If you are just tootling around the Southern Broads then I would go for the two cylinder model, if you are going to go to the Northern Broads then I would go for the larger three cylinder if only to pass through the Yarmouth bridges.

Regards

Alan

[LadyPatricia]

Strowager, I'm leaning towards the 21hp as the Volvo penta currently fitted is a 23hp. unfortunately we can't get the parts needed to repair it so it's either pay out for new engine or scrap the boat. 

 

Alan, we do like the odd trip north and seeing as she will go under all bridges we do want the option to use all of the rivers . 

 

Steve

[Guest]

8 minutes ago, LadyPatricia said:

..... we do like the odd trip north and seeing as she will go under all bridges we do want the option to use all of the rivers.....

I didn't know that you're based on the Southern rivers, but in that case I'd definitely go for the 21hp.

When the currents are adverse down there, they're generally much stronger to overcome than on most of the Northern rivers

 

[diesel falcon]

Hi

i d go for the larger unit also when you are plodding around normally you can use less revs ,less noise and as mentioned above it takes at lot of grunt to push assist a spring tide thou Yarmouth 

 

[Hockham Admiral]

Our Freeward 30 had 3.24 tons of pig iron encapsulated within the full length keel. With full water, fuel and the kitchen sink she tipped the scales at about 7 tons... definitely a heavy displacement boat.

The engine was a Sabb 2GZ with a maximum of 22bhp.

http://www.boatservicehaarlem.nl/sabb_2g_2j.pdf

With this boat we frequently ventured across the North Sea and down to the Channel Islands.....

 

[Guest]

A lovely looking boat John.

But she had a true displacement hull, designed to push through the water as efficiently as possible at displacement speeds, rather than starting to try to climb up onto it, to achieve planing speeds, thus needing much less horsepower.

(and you had Sails as well, the best and safest  "twin power" arrangement possible)

[JennyMorgan]

We sometimes cross Breydon with a 3.5 hp on a 28ft hull, just use the tides! John's example of his Freeward, a boat I always hankered after, is apt, in my opinion. I really don't see the need to over engine a boat on the off chance of a foul spring tide on Breydon, it's what tide tables are for!! I used to have a 7 hp on my Drascombe until, like Steve's, it died due to lack of spare parts. I asked other owners, all of which advised a smaller engine. Not only do I now have a lighter engine on the transom, with its obvious advantages, but my fuel bill is appreciably lighter!

[Guest]

4 minutes ago, JennyMorgan said:

We sometimes cross Breydon with a 3.5 hp on a 28ft hull, just use the tides! John's example of his Freeward, a boat I always hankered after, is apt, in my opinion. I really don't see the need to over engine a boat on the off chance of a foul spring tide on Breydon, it's what tide tables are for!! I used to have a 7 hp on my Drascombe until, like Steve's, it died due to lack of spare parts. I asked other owners, all of which advised a smaller engine. Not only do I now have a lighter engine on the transom, with its obvious advantages, but my fuel bill is appreciably lighter!

...and which of those examples had a semi-displacement or planing hull Peter ?

[JennyMorgan]

Effectively none? Although both a Drascombe and a Norman can plane I suspect that at legal Broads speeds neither can thus both are effectively semi-displacement.

[Guest]

22 minutes ago, JennyMorgan said:

Effectively none? Although both a Drascombe and a Norman can plane I suspect that at legal Broads speeds neither can thus both are effectively semi-displacement.

No Peter, a Drascombe definitely cannot plane. It is a full displacement hull.

You could put a 200hp outboard on the back, and it would tilt up onto its bow wave, (being unable to "plane" up onto it), until it sank.

As for the Norman, the Broads speed limit is precisely why the semi planing hull is less efficient for it, power-wise. Even at 6 knots, power (thrust) is being wasted by that hull shape trying to lift it up onto the surface, rather than just pushing through it, like your Drascombe.

The science of Displacement, Semi Displacement, and Planing hulls makes each type quite different in their thrust requirements, even at low speeds.

[JawsOrca]

Hi strowager. Just so I understand this. A semi planning hull needs more power than a displacement hull because power is wasted because it tries to rise up? And it tries to rise up even at displacement speeds? I would have thought at displacement speeds it's acting as a displacement hull (but I've not read too much into this). Interesting  though.

Steve certainly sounds like lady please needs the 3 cylinder one :(

[MauriceMynah]

Peter, one cannot always use the tides. Sadly sometimes the timing is such that you have no practical option other than to fight them.

My call would be for the larger engine option

[JennyMorgan]

Strowager, I can assure you that my Drascombe can and does plane, at least under sail! With its long run aft and deep foot forward it is most definitely a semi displacement hull. As for planing under motor, can't see the attraction myself!

[Guest]

1 hour ago, AlanHathaway said:

Hi strowager. Just so I understand this. A semi planning hull needs more power than a displacement hull because power is wasted because it tries to rise up? And it tries to rise up even at displacement speeds? I would have thought at displacement speeds it's acting as a displacement hull (but I've not read too much into this). Interesting  though.

Steve certainly sounds like lady please needs the 3 cylinder one

Yes Alan, even at displacement speeds, a planing or semi-planing hull needs is less efficient than a true displacement hull because of it's design.

In order to be able to plane if they're given sufficient power, planing hulls have a fuller transom, that is further down below the water surface at slow speeds.

Displacement hulls however, have a much cleaner "exit" at the transom, so that they cause less drag at low speeds.

Peter's Drascombe has a pointed stern, so that is the most efficient displacement profile, like a canoe. Your Norman however, has a typical motor cruiser "boxy" transom that creates a lot more resistance when the water tries to fill the gap after the hull has passed.

It's not very noticeable when they're afloat, but even when a full displacement motor cruiser has a square transom, very little of it extends below the water line.

 

[JawsOrca]

Thanks strowager very interesting indeed! Cheers

[Guest]

2 hours ago, JennyMorgan said:

Strowager, I can assure you that my Drascombe can and does plane, at least under sail! With its long run aft and deep foot forward it is most definitely a semi displacement hull. As for planing under motor, can't see the attraction myself!

I still beg to differ Peter.

The Drascombe Lugger is most definitely a true displacement hull, in fact it is probably one of the most efficient examples of one, hence it's minimal auxiliary power requirement.

If you still don't believe it, have a look at this unusually tailor made PDF, which contains the sentence "The Drascombe hulls are essentially displacement types, which  means that they are shaped to move through the water pushing the water aside as they  go along.  This is the most efficient way to travel at sailing speeds."

http://www.yacht-designer.co.uk/20-knotDrascombeText.pdf

(and it was written by Stuart M Roy, a Naval Architect and Yacht Designer)

 

[Guest]

Sorry chaps, but I've just found another bug in the software.

When I made that last post, it included the image from my previous post, even though I hadn't linked it.

When I then removed the image from that post, it removed it from the other one too.

So this image belongs with my post made at 12:10....

[JennyMorgan]

I will agree that many Drascombe owners treat their boats as 'essentially' displacement hulls but the reality is that they are capable of planing. Indeed another naval architect has debated the potential of a 50 knot Drascombe. By the way, my Drascombe is a Longboat.

[BroadScot]

6 minutes ago, JennyMorgan said:

 By the way, my Drascombe is a Longboat.

Oh JM do you dress up in all the VIKING clobber in your Longboat?  Tony Curtis eat yer heart out!

Iain.

[JennyMorgan]

I thought that I had on file the sections of a Drascombe Lugger but unfortunately that file must be on another computer. Had I been able to find it then it would have shown quite clearly that my boat does not have a pointed stern. The Drascombe Scaffie does but both the Lugger and Longboat have flat transoms and near flat aft sections, hence their ability to surf, to plane and for the boat to be beached stern first thus allowing the crew to step ashore, often dry shod.