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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pioneer Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2007 at 9:40am

Hello Christian

I remember reading somewhere that there is a formula for determining a vessels expected top speed (something along the lines of loa x weight x beam plus displacement etc and of course the power available) 50 Knts from just under 4000hp would be stretching the figures I would guess - horses mouths? (I once lost a packet listening to them)

Best Regards

Ted

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Post Options Post Options   Thanks (0) Thanks(0)   Quote d-zine Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2007 at 12:12pm
Hi Christian and Pioneer,

I have been using a Prop/Engine/Gearbox sizing program to tryout various engine gearbox arrangments for 2753. As a check to see how accurate the software was I used known data from when the RTTL's were in service.
I based the data on RR Griffons x 2 at 1800bhp at 2100rpm. The V drives had a 1.54:1 reduction and the props were 34" x 37".
Assuming a very efficient planing hull (which of course it is) the program calculated a slippage of 0.11 ie 89% efficient. I based the final calculation on a displacent of 39 tons which is a fair estimate of service displacement. The result is a service speed of 39 knots which ties in with the claimed service speed.

I then made the same calculation based on a displacement of 15 tons which is what I estimate 2753 will be when engined ( the same program estimates her current displacement at 9 tons based on draft-admittedly this is a real estimate). With the same props and 3600bhp the estimated maximum speed is 62knots/72mph.

It's not unreasonable to imagine these boats in service acheiving 45 knots+.
I wonder whether Vosper might have test data for these craft or 1601 which was the same hull form?


Edited by d-zine
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Christian Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2007 at 12:33pm

Hi d-zine

Funnily enough I bumped into one of my contacts today, I asked him if he was sure about what he'd told me and he was adamant. He said they revved the engines "over 2200 rpm" to achieve these speeds.  

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Post Options Post Options   Thanks (0) Thanks(0)   Quote d-zine Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2007 at 1:30pm
Hi Christian,

Refering to the Boat Histories section of the site the Article on the Braves describes the hull design as being based on 1601 (RTTL) form and that was capable of speeds in excess of 50 knots with the gas turbine installation.
Obvoiusly direct comparisons cannot be drawn, however it would appear that the hull form is perfectly capable of these speeds subject to weight and power.
Reference is made to the admiralty carrying out extensive trials in the Haslar Test Tank, I am sure that these would provide evidence of the theoretical maximum speed for this type of craft.

I would love to see one of these old warriors at something approaching top speed.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Christian Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2007 at 2:57pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Christian Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2007 at 3:11pm

From Ray, who served on them;

Hi christian,
                      All the info I can remember was a boat with griffons reached 45 knots but with a lot of vibration from the back end, I would like to have been on board
 
 
 
 
And from Donald Smith, RAF Marine Craft Historian;
 
Regarding the top speed of the RTTLs, with a dry boat just off the slip 41-41 knots was achievable, until the hull soaked up water and they reverted to 39 knots.  We had one boat 2762E later 2772E and finally 2772 which originally could touch 50 knots. This was before her bottom was re-skinned with mahogany which slowed her down to a more standard performance.
 
Hats off to Ted.
Rgds, Christian.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pioneer Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2007 at 5:33pm

Hello Christian and d-zine,

I am one of those unfortunate people that has a 'cup half empty' as opposed to one being 'half full' - I'm constantly told by my better half. - 1601 was an experimental MTB (not an RTTL) the Hull being similar in form derived from the Vosper Launch 'Swordfish'  The Hull's capabilty is not in question but the amount of power required to drive it to 50 Knts is beyond the delivery of twin Griffons I still believe. Although we are talking of completely different vessels - the 'Braves had 3 x 3,600hp GT's for its superb performance figures while 'Ferocity' had 2 x up-rated GT's (4200hp) and even that did not quite match a Brave (but very close indeed).

Regards Ted

 



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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMSmith Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2007 at 10:00pm

Christian

Thanks for the helping me again, this time saving me a retun trip. (Although I'll go back sometime as I'm only an hour or so from Hendon.) Is 2757 the only complete one left?

Fascinated by the discussion on top speed, guess the question is what sort of log was used?

Cheers

Dave

 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote d-zine Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2007 at 10:14pm

Hi Dave,

2748 is nearing the completion of a comprehensive restoration.(Plymouth)

2751 is also under restoration and looks quite close to original appearance( Guernsey)

2753 is close to original but missing various bits and pieces(Gibraltar)

2757 is the Hendon boat

2758 has undergone a complete conversion to a very smart looking motor yacht.(Malta)

2768 when last seen looked original but in very poor condition (Malta).

2770 has had her wheel house extended aft but is in otherwise good condition.(Dumbarton)

2771 when last seen was a hulk (Malta).

There should be pictures posted elsewhere on the site for most of the boats mentioned above.

2754 sank in 1975/6 some way off Gibraltar.

2762/2772e was BU in Middlesborough in 1996 I think.

As for the others information would be welcome

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Christian Quote  Post ReplyReply Direct Link To This Post Posted: 01 March 2007 at 11:09pm

Hi Dave

They used the venerable and accurate Chernikeef logs.

Hi d-zine

All I can add is 2760 was at Hooe Lake, Plymouth until BU mid 90s. Also suspected Vosper RTTL washed ashore by Asian tsunami, still unconfirmed though.

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