5 m Fast Rescue Boat Design - Comments and Suggestions

Hi all

I am currently designing a rescue boat with particulars;

L 5 m
B 2 m
T 0.25 m

using DelftShip Free version. I want the displacement to be around 650-700 kg. The boat is a rescue boat so it must go at high speeds on rough weather conditions. I am waiting your comments and suggestions on how to make the boat more seaworthy and fast.

I am also asking your suggestions on the 2 parts at the aft section above the waterline. I made them to have extra buoyancy on idle condition and also to protect the outboard engine. Do you think its right to make such extruding parts? what could happen when turning at high speeds? Because i moved the transom forward, i have less buoyancy then i should have. Its around 600 kg. Should i remove those parts and pull the transom back at station 0?

I appreciate any suggestions and comments. Thank you in advance.

John


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John,

Is there a budgetary reason for you designing (and supposedly building) a boat for this purpose yourself?

Because if there isn't, why not simply buy a good RIB? Seaworthy, good in rough weather, fast, stable and light.

 

why not RIB? cause they get holes from sharp objects &

by definition aren't the best hull shape?

He seems to be going for far narrower than any similar RIB.

 

I beg to differ on both points. The running surfaces of most RIB's are quite narrow for their length and they are widely used throughout the rescue and military fields for their ability to perform in the worst of conditions. The inflatable collar may have a shorter lifespan than some other materials, but it softens impacts, improving the life of both vessels and crew.

 

From my experience, rescue boats are either RIB's or closed cabin self-righting vessels (with an inboard). Both unsinkable (in theory of course).

The big advantage of the RIB is usually it's speed if the conditions are rough, but not extreme, the work space and the ability for crew to move around and pull swimmers aboard. The inflatable collar has some plusses in that field.

The advantage of the closed cabin versions is of course shelter.

I have yet to see the advantage of an open boat like John is suggesting over any of the common rescue vessels.

The resue field is one with a lot of dedicated professionals (my mum used to be one ), who generaly know very well what they want and need, so if you come with something new, it better be good, and you need to know what you're talking about.

 

A rescue boat, (for ships) shall be tested with a sideways stroke, fully loaded. A drop test, fully loaded. And afterwards it shall be operatable, business as usual. No requirement to be unharmed, but it shall not cause the boat to be unable to perform.

A rib has a fender that can take a lot of punishment, also not in the water under normal situations, making it (the hull) able to perform well. There are options for fenders, foam and "glued" together and covered, that also provide superb protection and bouyancy. Big fenders like this or as for RIBs offer a very stable platform, with minimum height to pull out people in the water.

The two sides you have made on the hull, aft of the engine; I would make this at an angle towards the transom area to make the stress (during the sideways stoke test) "diffuce" into the transom, without causing the inner corner to crush.

Another thing with the two sides aft; I would let the bottom of the boat follow these, making them "work" when boat is at speed (or perfoming the liferaft test/ bollard pull)

Also; two men in the water shall be able to righten an inverted boat, if the boat is not self rightening, thats the fun part to test...

 

first of all thanks for all your comments.

the boat will be open and will have an apparatus (lever or whatever its called) at the aft and i will make sufficient buoyancy tanks filled with poliuretan so that the boat will float when its fully loaded with wather. There will also be a rubber ribbon around the boat to absorbe the shocks. (of corse it wont be as good as the inflatable collar around the rib)

Knut, i will make an angle between the transom and the two sides as you suggested. As to make the two parts follow the hulls lines, dont you think a turning moment would be formed if the wather hit those two parts while turning fast?

I also changed the chines lines to have a varying deadrise so that the boat is more seaworthy in bad weather..

 

Didn't think of that.... can make it harder to turn... maybe somewhat at least, but if you make it at an angle as you said, the boat will during turning, also be moving forward, well normally, so it may also just act partly as a rudder... Didn't think of that... may be a problem in slow speed, and full "rudder", and sometimes, at least, I myself, prefer to do just that.. Standstill, full "rudder", and hit the throttle, just to "jump" the boat around. With an outboard, the propeller should normally be low enough to anyway be able to give the boat that kick.

Saw a photocopy of a known boat, also a ..."copy", they had transformed a boat I recognized (Outboard, this actual hull worked ok with an outboard, and had a similar hull, but I think there should be some thought behind how far down you'd pull the bottom), into a FRB waterjet, had just this shape at the rear. Boss said; "find one error, no sigar"... the sides of the hull protected the protruding waterjet, but it also fenced in the steering ability of the waterjet....

If you check http://www.norsafe.no/index.php?pid=1336&aid=1750 you may get some ideas.

btw1; for a fender; http://www.norsafe.no/index.php?pid=1336&aid=2453
The fender here is first glued layers of pretty hard foam, cut to shape screwed layer of heavy duty rubber cloth, and then an extra even more heavy duty rubber protection.

btw2; The last boat there is a beauty... rides well, handles easy, low freeboard.

btw3; 5 m is not too long a boat for this task....

 

sorry i havent replied to the comments for long. but thank you very much for the suggestions. there is a new IMO Regulation that says rescue boats cannot be smaller than 6 meters. so i had to extend the lenght of the boat to that. and i had to remove the aft parts to make the waterline actually longer (with full breath boyancy till the aft). because the boat is going to be 1500 kg with 3 crew members (each 82.5 kg). The naked hull will be around 600 kg including the deck and self righting arm.. the rest is engine tank solas lifting hook and its leg and so and so.. anyways the boat has enough displacement at 35 cm draft. im just scared that the V hull is not deep V enough. the deadrise is 11 degrees at transom. and 15 at station 7. so the hull is flatter at the aft. that will help me to get on planing with a smaller engine but do you think it will pound alot in rough water? sorry i asked more questions trying to explain what i am doing. again i appretiate any comment

 

A deep vee will (normally) help for pounding, give a better ability for operation in rough seas. On the downside; you'll need more power for a certain amount of speed. Win some, you loose some...

A rescue boat is not, normally designed for a comfort ride...

One of the biggest problem with a flatter bottom, is the "drop test", fully loaded, 3 m, no significant damage to the intended operation.

On the models posted, make as many rounded corners as possible (only the lower part of the transom needs to be sharp to slip the water... Rounded corners will dissipate the stress, sharp corners will break/ crush. I'm thinking of the sideways stroke test...

The new rules? did they add a requirement for max g-force during the drop test? I know that the issue has been discussed... A too flat bottomed boat will probably not meet any requirement regarding that (wellll, builditheavy...?)

 

With a long experience from this field I can hopefully help. These are the important things you need to think of first (unless you already haven’t) before any design really make sense.

What kind of rescue operations and circumstances is the boat supposed to operate under?
What’s the maximum mission response time?

The first question is about painting the scenarios that this vessel will operate. Day, night, weather and waves. What kind of rescue missions will it undertake (what has happened)? How many crews do you need to handle those situations? What gear do you need to carry etc?

After answering that you can go on and figure out how fast you need to be there. What’s your planned response time? - From alerted, how long can it take to get it operational? Where will it be stationed (on a trailer, ship or other)? How large is the mission area and how fast do you need to be able to cover it?

Not before you have those basic questions answered you can start to figure out what kind of equipment you need.