Hi, this is my first post. I am at the planning stage of building a wooden dingy based on the International 505. I will be building this using strip plank + veneer or all veneer with wooden spars. Its not my first build and I know there is lots of work but I have a good workshop and tools.
I've considered an Argie 15 and an Iain Oughtred Gannet but in the end I've chosen the 505 as a base because I like its shape, performance and righting-after-capsize ability (built in flotation at the rail - reduced bailing, no rescue craft here!) and, also, I have the plans for it.
I have two main reasons for scaling down. The first is because my wife and I want to be able to handle the craft both in and out of the water and 5.05 metres may be a bit of a handful for us. This is especially true because we are based in Suzhou in China and will have a degree of man-handling, though we will use a trailer of sorts. (this is because sailing in China is virtually unknown except in one or two coastal centres, so there are no slip-ways in our local lakes.) The second reason for scaling down is to slightly de-power the craft because, although I am fairly experienced, I want to sail with my wife and some friends who are (so far) not experienced.
I've done a fair bit of reading on the internet and also studied all the relevant bits in Dave Gerr's "The Nature of Boats". Following this, I've scaled down the plans using Autocad and "Freeship" for calculation of some of the hydrostatics.
The scaling I've used is not uniform. I understand and have read all I can find on scaling similitude For instance, I've scaled the length by 0.83 and the beam and profile by 0.91. I've attached a set of metrics.
There are two things I'm a bit worried about:
The first is the Lead. This is about 290mm in the 505 and is about 350mm in my scaled version. This is the only thing that's gone up in scaling and is because despite reducing mast height, I've raised (relatively) the head of the jib which increases its size and therefore moves the CP forward. Two reasons for this: firstly because I was worried about it being underpowered and secondly because I think that the increase in curvature of the hull (resulting from the non-uniform scaling) will increase the lee helm so I felt the lead should be increased a bit.
The second concern is the power. Have I powered it down too much, or not-enough? We still want to be able to get up on a plane without sailing in harsh winds. As far as I can see from the theory, it will plane at about 8Kts I've done some power and heeling calculations but its all a bit theoretical and back-of-a-fag-packet.
My table of scaling factors is shown below. I'd be really grateful for any constructive input, not only on my concerns but also on anything you feel I should change or do more research on.
Sorry the table doesn't format well in the post. There a three columns: Original 505, New and Scale Factor.
Dimensions in mm -------------- Original 505 New Scale Factor
Length OA --------------------------- 5050 4200 0.83
Length WL --------------------------- 4500 4000 0.89
Beam -------------------------------- 1860 1700 0.91
Draft --------------------------------- 147 134 0.91
Disp with 2 crew (Kg)--------------- 257 250 0.97
Mast height -------------------------- 7260 6000 0.83
Boom length ------------------------ 3120 2590 0.83
Mainsail area sq m ------------------ 11.74 7.87 0.67
Jib area sq m ------------------------ 4.68 3.9 0.83
Total sail area sq m ----------------- 16.42 11.77 0.72
SA/D^2/3 ---------------------------- 37 27 0.73
Height of CP from w/l --------------- 2494 2263 0.91
Lead (CP fwd of CLR) --------------- 290 350 1.21
Horsepower at F3-4 ---------------- 3.35 2.40 0.72
I've considered an Argie 15 and an Iain Oughtred Gannet but in the end I've chosen the 505 as a base because I like its shape, performance and righting-after-capsize ability (built in flotation at the rail - reduced bailing, no rescue craft here!) and, also, I have the plans for it.
I have two main reasons for scaling down. The first is because my wife and I want to be able to handle the craft both in and out of the water and 5.05 metres may be a bit of a handful for us. This is especially true because we are based in Suzhou in China and will have a degree of man-handling, though we will use a trailer of sorts. (this is because sailing in China is virtually unknown except in one or two coastal centres, so there are no slip-ways in our local lakes.) The second reason for scaling down is to slightly de-power the craft because, although I am fairly experienced, I want to sail with my wife and some friends who are (so far) not experienced.
I've done a fair bit of reading on the internet and also studied all the relevant bits in Dave Gerr's "The Nature of Boats". Following this, I've scaled down the plans using Autocad and "Freeship" for calculation of some of the hydrostatics.
The scaling I've used is not uniform. I understand and have read all I can find on scaling similitude For instance, I've scaled the length by 0.83 and the beam and profile by 0.91. I've attached a set of metrics.
There are two things I'm a bit worried about:
The first is the Lead. This is about 290mm in the 505 and is about 350mm in my scaled version. This is the only thing that's gone up in scaling and is because despite reducing mast height, I've raised (relatively) the head of the jib which increases its size and therefore moves the CP forward. Two reasons for this: firstly because I was worried about it being underpowered and secondly because I think that the increase in curvature of the hull (resulting from the non-uniform scaling) will increase the lee helm so I felt the lead should be increased a bit.
The second concern is the power. Have I powered it down too much, or not-enough? We still want to be able to get up on a plane without sailing in harsh winds. As far as I can see from the theory, it will plane at about 8Kts I've done some power and heeling calculations but its all a bit theoretical and back-of-a-fag-packet.
My table of scaling factors is shown below. I'd be really grateful for any constructive input, not only on my concerns but also on anything you feel I should change or do more research on.
Sorry the table doesn't format well in the post. There a three columns: Original 505, New and Scale Factor.
Dimensions in mm -------------- Original 505 New Scale Factor
Length OA --------------------------- 5050 4200 0.83
Length WL --------------------------- 4500 4000 0.89
Beam -------------------------------- 1860 1700 0.91
Draft --------------------------------- 147 134 0.91
Disp with 2 crew (Kg)--------------- 257 250 0.97
Mast height -------------------------- 7260 6000 0.83
Boom length ------------------------ 3120 2590 0.83
Mainsail area sq m ------------------ 11.74 7.87 0.67
Jib area sq m ------------------------ 4.68 3.9 0.83
Total sail area sq m ----------------- 16.42 11.77 0.72
SA/D^2/3 ---------------------------- 37 27 0.73
Height of CP from w/l --------------- 2494 2263 0.91
Lead (CP fwd of CLR) --------------- 290 350 1.21
Horsepower at F3-4 ---------------- 3.35 2.40 0.72
